[BUG] ICE in clang::CodeGen::CodeGenFunction::EmitAsmStmt(clang::AsmStmt const&)

## Bug Report: ICE in clang::CodeGen::CodeGenFunction::EmitAsmStmt(clang::AsmStmt const&)

Clang crashes when compiling the following C code.

**Compiler**: clang-22 (LLVM 22.1.5, commit 5ea218a153f4d2f815b8244eab3e4b4ba5e00e6c)
**Compilation flags**: -O0
**Platform**: x86_64-linux-gnu
**Component**: frontend

This is a **frontend** crash (reproduces with `-emit-llvm -Xclang -disable-llvm-passes`).

## Reduced Test Case

> Minimized from the original fuzzer-generated input using `creduce`.

```sh
clang -O0 test.c -o /dev/null
```

```c
typedef struct {
} myjmp_buf;
typedef struct {
myjmp_buf regs;
} my_stack;
void switch_to_stack (my_stack *stack){
asm volatile (
"movq %0, %%r11; movq $20216, %%rcx\n"
"lfence\n"
"syscall;\n"
"\n"
: "+r" (stack->regs)
);
}
```

## Crash Output

```
/root/outputs/experiments-on-metamut/surfuzz,gcc,3/BDiklssbvzuL_surfuzz_29.c:12:24: error: output size should not be zero
12 | : "+r" (stack->regs)
| ^
Failure value returned from cantFail wrapped call
inline asm with one output cannot return struct
UNREACHABLE executed at /root/compilers/llvm-project-release/llvm/include/llvm/Support/Error.h:779!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0\. Program arguments: /root/objects/llvm-project-release.obj/bin/clang-22 -cc1 -triple x86_64-unknown-linux-gnu -O0 -emit-obj -dumpdir /dev/null- -disable-free -clear-ast-before-backend -main-file-name BDiklssbvzuL_surfuzz_29.c -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=all -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/root/data -fcoverage-compilation-dir=/root/data -resource-dir /root/objects/llvm-project-release.obj/lib/clang/22 -internal-isystem /root/objects/llvm-project-release.obj/lib/clang/22/include -internal-isystem /usr/local/include -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -ferror-limit 19 -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -faddrsig -fdwarf2-cfi-asm -o /tmp/BDiklssbvzuL_surfuzz_29-8b8506.o -x c /root/outputs/experiments-on-metamut/surfuzz,gcc,3/BDiklssbvzuL_surfuzz_29.c
1\. parser at end of file
2\. /root/outputs/experiments-on-metamut/surfuzz,gcc,3/BDiklssbvzuL_surfuzz_29.c:6:6: LLVM IR generation of declaration 'switch_to_stack'
3\. /root/outputs/experiments-on-metamut/surfuzz,gcc,3/BDiklssbvzuL_surfuzz_29.c:6:6: Generating code for declaration 'switch_to_stack'
#0 0x00005569478f82d2 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/root/objects/llvm-project-release.obj/bin/clang-22+0x44f62d2)
#1 0x00005569478f4caf llvm::sys::RunSignalHandlers() (/root/objects/llvm-project-release.obj/bin/clang-22+0x44f2caf)
#2 0x00005569478f4dfc SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
#3 0x00007f3c482d6520 (/usr/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#4 0x00007f3c4832aa7c __pthread_kill_implementation ./nptl/./nptl/pthread_kill.c:44:76
#5 0x00007f3c4832aa7c __pthread_kill_internal ./nptl/./nptl/pthread_kill.c:78:10
#6 0x00007f3c4832aa7c pthread_kill ./nptl/./nptl/pthread_kill.c:89:10
#7 0x00007f3c482d6476 gsignal ./signal/../sysdeps/posix/raise.c:27:6
#8 0x00007f3c482bc7f3 abort ./stdlib/./stdlib/abort.c:81:7
#9 0x0000556947836b5e (/root/objects/llvm-project-release.obj/bin/clang-22+0x4434b5e)
#10 0x0000556947162088 llvm::InlineAsm::InlineAsm(llvm::FunctionType*, std::__cxx11::basic_string, std::allocator > const&, std::__cxx11::basic_string, std::allocator > const&, bool, bool, llvm::InlineAsm::AsmDialect, bool) (/root/objects/llvm-project-release.obj/bin/clang-22+0x3d60088)
#11 0x0000556947162459 llvm::ConstantUniqueMap::getOrCreate(llvm::PointerType*, llvm::InlineAsmKeyType) (/root/objects/llvm-project-release.obj/bin/clang-22+0x3d60459)
#12 0x00005569471631eb llvm::InlineAsm::get(llvm::FunctionType*, llvm::StringRef, llvm::StringRef, bool, bool, llvm::InlineAsm::AsmDialect, bool) (/root/objects/llvm-project-release.obj/bin/clang-22+0x3d611eb)
#13 0x0000556947dfd5fc clang::CodeGen::CodeGenFunction::EmitAsmStmt(clang::AsmStmt const&) (/root/objects/llvm-project-release.obj/bin/clang-22+0x49fb5fc)
#14 0x0000556947e0978b clang::CodeGen::CodeGenFunction::EmitCompoundStmtWithoutScope(clang::CompoundStmt const&, bool, clang::CodeGen::AggValueSlot) (/root/objects/llvm-project-release.obj/bin/clang-22+0x4a0778b)
#15 0x0000556947e719e3 clang::CodeGen::CodeGenFunction::EmitFunctionBody(clang::Stmt const*) (/root/objects/llvm-project-release.obj/bin/clang-22+0x4a6f9e3)
#16 0x0000556947e86e84 clang::CodeGen::CodeGenFunction::GenerateCode(clang::GlobalDecl, llvm::Function*, clang::CodeGen::CGFunctionInfo const&) (/root/objects/llvm-project-release.obj/bin/clang-22+0x4a84e84)
#17 0x0000556947eeaf0d clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/root/objects/llvm-project-release.obj/bin/clang-22+0x4ae8f0d)
#18 0x0000556947ee5ef5 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/root/objects/llvm-project-release.obj/bin/clang-22+0x4ae3ef5)
#19 0x0000556947ee6b83 clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) (/root/objects/llvm-project-release.obj/bin/clang-22+0x4ae4b83)
#20 0x0000556947ef0807 clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) (.part.0) CodeGenModule.cpp:0:0
#21 0x000055694826cd71 (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) ModuleBuilder.cpp:0:0
#22 0x000055694825dcc1 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (/root/objects/llvm-project-release.obj/bin/clang-22+0x4e5bcc1)
#23 0x000055694a109874 clang::ParseAST(clang::Sema&, bool, bool) (/root/objects/llvm-project-release.obj/bin/clang-22+0x6d07874)
#24 0x0000556948581987 clang::FrontendAction::Execute() (/root/objects/llvm-project-release.obj/bin/clang-22+0x517f987)
#25 0x00005569485090e9 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/root/objects/llvm-project-release.obj/bin/clang-22+0x51070e9)
#26 0x000055694867e95e clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/root/objects/llvm-project-release.obj/bin/clang-22+0x527c95e)
#27 0x0000556944389657 cc1_main(llvm::ArrayRef, char const*, void*) (/root/objects/llvm-project-release.obj/bin/clang-22+0xf87657)
#28 0x000055694437e694 ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#29 0x0000556944383907 clang_main(int, char**, llvm::ToolContext const&) (/root/objects/llvm-project-release.obj/bin/clang-22+0xf81907)
#30 0x00005569442410ea main (/root/objects/llvm-project-release.obj/bin/clang-22+0xe3f0ea)
#31 0x00007f3c482bdd90 __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:58:16
#32 0x00007f3c482bde40 call_init ./csu/../csu/libc-start.c:128:20
#33 0x00007f3c482bde40 __libc_start_main ./csu/../csu/libc-start.c:379:5
#34 0x000055694437dcb5 _start (/root/objects/llvm-project-release.obj/bin/clang-22+0xf7bcb5)
clang: error: unable to execute command: Aborted (core dumped)
clang: error: clang frontend command failed due to signal (use -v to see invocation)
clang version 22.1.5 (https://github.com/llvm/llvm-project.git 5ea218a153f4d2f815b8244eab3e4b4ba5e00e6c)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /root/objects/llvm-project-release.obj/bin
Build config: \+assertions
clang: note: diagnostic msg:
********************
PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang: note: diagnostic msg: /tmp/BDiklssbvzuL_surfuzz_29-c933c0.c
clang: note: diagnostic msg: /tmp/BDiklssbvzuL_surfuzz_29-c933c0.sh
clang: note: diagnostic msg:
********************
EXIT_CODE: 1
```

## Preprocessed Source & Run Script

**Preprocessed source** (`/tmp/BDiklssbvzuL_surfuzz_29-c933c0.c`):

```c
# 1 ""
# 1 "/root/outputs/experiments-on-metamut/surfuzz,gcc,3/BDiklssbvzuL_surfuzz_29.c"
typedef struct {
} myjmp_buf;
typedef struct {
myjmp_buf regs;
} my_stack;
void switch_to_stack (my_stack *stack){
asm volatile (
"movq %0, %%r11; movq $20216, %%rcx\n"
"lfence\n"
"syscall;\n"
"\n"
: "+r" (stack->regs)
);
}

```

**Run script** (`/tmp/BDiklssbvzuL_surfuzz_29-c933c0.sh`):

```sh
# Crash reproducer for clang version 22.1.5 (https://github.com/llvm/llvm-project.git 5ea218a153f4d2f815b8244eab3e4b4ba5e00e6c)
# Driver args: "-O0" "/root/outputs/experiments-on-metamut/surfuzz,gcc,3/BDiklssbvzuL_surfuzz_29.c" "-o" "/dev/null"
# Original command: "/root/objects/llvm-project-release.obj/bin/clang-22" "-cc1" "-triple" "x86_64-unknown-linux-gnu" "-O0" "-emit-obj" "-dumpdir" "/dev/null-" "-disable-free" "-clear-ast-before-backend" "-main-file-name" "BDiklssbvzuL_surfuzz_29.c" "-mrelocation-model" "pic" "-pic-level" "2" "-pic-is-pie" "-mframe-pointer=all" "-fmath-errno" "-ffp-contract=on" "-fno-rounding-math" "-mconstructor-aliases" "-funwind-tables=2" "-target-cpu" "x86-64" "-tune-cpu" "generic" "-debugger-tuning=gdb" "-fdebug-compilation-dir=/root/data" "-fcoverage-compilation-dir=/root/data" "-resource-dir" "/root/objects/llvm-project-release.obj/lib/clang/22" "-internal-isystem" "/root/objects/llvm-project-release.obj/lib/clang/22/include" "-internal-isystem" "/usr/local/include" "-internal-isystem" "/usr/lib/gcc/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/include" "-internal-externc-isystem" "/usr/include/x86_64-linux-gnu" "-internal-externc-isystem" "/include" "-internal-externc-isystem" "/usr/include" "-ferror-limit" "19" "-fgnuc-version=4.2.1" "-fskip-odr-check-in-gmf" "-faddrsig" "-fdwarf2-cfi-asm" "-o" "/tmp/BDiklssbvzuL_surfuzz_29-8b8506.o" "-x" "c" "/root/outputs/experiments-on-metamut/surfuzz,gcc,3/BDiklssbvzuL_surfuzz_29.c"
"/root/objects/llvm-project-release.obj/bin/clang-22" "-cc1" "-triple" "x86_64-unknown-linux-gnu" "-O0" "-emit-obj" "-dumpdir" "/dev/null-" "-disable-free" "-clear-ast-before-backend" "-main-file-name" "BDiklssbvzuL_surfuzz_29.c" "-mrelocation-model" "pic" "-pic-level" "2" "-pic-is-pie" "-mframe-pointer=all" "-fmath-errno" "-ffp-contract=on" "-fno-rounding-math" "-mconstructor-aliases" "-funwind-tables=2" "-target-cpu" "x86-64" "-tune-cpu" "generic" "-debugger-tuning=gdb" "-fdebug-compilation-dir=/root/data" "-fcoverage-compilation-dir=/root/data" "-ferror-limit" "19" "-fgnuc-version=4.2.1" "-fskip-odr-check-in-gmf" "-faddrsig" "-fdwarf2-cfi-asm" "-x" "c" "BDiklssbvzuL_surfuzz_29-c933c0.c"

```

* found via a fuzzer

[BDiklssbvzuL_surfuzz_29-c933c0.c](https://github.com/user-attachments/files/29127577/BDiklssbvzuL_surfuzz_29-c933c0.c)
[BDiklssbvzuL_surfuzz_29-c933c0.sh](https://github.com/user-attachments/files/29127578/BDiklssbvzuL_surfuzz_29-c933c0.sh)

Start by running the provided clang -O0 command with the reduced C source and inspect CodeGenFunction::EmitAsmStmt, the stack-trace entry point named in the report. Compare the inline-assembly handling of the zero-sized struct operand and add a regression test based on the reproducer. Done means clang reports the invalid input without aborting or reaching the LLVM assertion.

Written by the indexing model from the issue text.