clang trunk: assertion "Must have at least one constraint" in TargetLowering::ComputeConstraintToUse during backend DAG for inline asm with malformed leading comma constraint

## Compiler Explorer: https://godbolt.org/z/r7E47n1xr

- clang start crashing on `Trunk`
- generated by fuzzer

## Console
```console
clang: /root/llvm-project/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp:6284: virtual void llvm::TargetLowering::ComputeConstraintToUse(llvm::TargetLowering::AsmOperandInfo&, llvm::SDValue, llvm::SelectionDAG*) const: Assertion `!OpInfo.Codes.empty() && "Must have at least one constraint"' failed.
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: /opt/compiler-explorer/clang-assertions-trunk/bin/clang -g -o /app/output.s -mllvm --x86-asm-syntax=intel -fno-verbose-asm -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics
1. parser at end of file
2. Code generation
3. Running pass 'Function Pass Manager' on module ''.
4. Running pass 'X86 DAG->DAG Instruction Selection' on function '@f3'
#0 0x00000000042ecab8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x42ecab8)
#1 0x00000000042e9ee4 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x42e9ee4)
#2 0x000000000422ec88 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#3 0x0000701477e42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#4 0x0000701477e969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#5 0x0000701477e42476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#6 0x0000701477e287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#7 0x0000701477e2871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#8 0x0000701477e39e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#9 0x00000000059d0faa llvm::TargetLowering::ComputeConstraintToUse(llvm::TargetLowering::AsmOperandInfo&, llvm::SDValue, llvm::SelectionDAG*) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x59d0faa)
#10 0x00000000058888f3 llvm::SelectionDAGBuilder::visitInlineAsm(llvm::CallBase const&, llvm::BasicBlock const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x58888f3)
#11 0x00000000058a7ccf llvm::SelectionDAGBuilder::visitCall(llvm::CallInst const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x58a7ccf)
#12 0x00000000058bc052 llvm::SelectionDAGBuilder::visit(llvm::Instruction const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x58bc052)
#13 0x000000000594eb1d llvm::SelectionDAGISel::SelectBasicBlock(llvm::ilist_iterator_w_bits, false, true>, llvm::ilist_iterator_w_bits, false, true>, bool&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x594eb1d)
#14 0x000000000594fa32 llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x594fa32)
#15 0x00000000059510bc llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x59510bc)
#16 0x000000000593c94f llvm::SelectionDAGISelLegacy::runOnMachineFunction(llvm::MachineFunction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x593c94f)
#17 0x00000000035843d9 llvm::MachineFunctionPass::runOnFunction(llvm::Function&) (.part.0) MachineFunctionPass.cpp:0:0
#18 0x0000000003bda986 llvm::FPPassManager::runOnFunction(llvm::Function&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x3bda986)
#19 0x0000000003bdad31 llvm::FPPassManager::runOnModule(llvm::Module&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x3bdad31)
#20 0x0000000003bdb59f llvm::legacy::PassManagerImpl::run(llvm::Module&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x3bdb59f)
#21 0x00000000045ae49c (anonymous namespace)::EmitAssemblyHelper::RunCodegenPipeline(clang::BackendAction, std::unique_ptr>&, std::unique_ptr>&) BackendUtil.cpp:0:0
#22 0x00000000045aedd0 clang::emitBackendOutput(clang::CompilerInstance&, clang::CodeGenOptions&, llvm::StringRef, llvm::Module*, clang::BackendAction, llvm::IntrusiveRefCntPtr, std::unique_ptr>, clang::BackendConsumer*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x45aedd0)
#23 0x0000000004c4d987 clang::BackendConsumer::HandleTranslationUnit(clang::ASTContext&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x4c4d987)
#24 0x0000000006a08fec clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x6a08fec)
#25 0x0000000004c4dde8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x4c4dde8)
#26 0x0000000004f446e5 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x4f446e5)
#27 0x0000000004ec497e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x4ec497e)
#28 0x000000000503ea6d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x503ea6d)
#29 0x0000000000de5b4c cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0xde5b4c)
#30 0x0000000000ddc59a ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#31 0x0000000000ddc71d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#32 0x0000000004cc0739 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#33 0x000000000422f124 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x422f124)
#34 0x0000000004cc0d4f clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#35 0x0000000004c81832 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x4c81832)
#36 0x0000000004c827de clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x4c827de)
#37 0x0000000004c89c35 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0x4c89c35)
#38 0x0000000000de1f41 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0xde1f41)
#39 0x0000000000c8cf94 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0xc8cf94)
#40 0x0000701477e29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#41 0x0000701477e29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#42 0x0000000000ddc035 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+0xddc035)
clang: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134
```

## Code
```c
int a[4];

int f1(int x, int y, int z) { return a[x] + (1 << y) + (100 / z); }

char *f2(int x) {
char *p = (char *)__builtin_calloc(64, 1);
p[x] = 3;
return p;
}

int f3(int x, int *y, double z, double w) {
int a[*y];
if (x)
__builtin_unreachable();
asm volatile("" : : ",r"(&a[0]));
return z / w;
}

int main() { return 0; }
```

Reproduce the crash with the C example and inspect llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp at TargetLowering::ComputeConstraintToUse, following the inline-asm path in the stack trace. Determine how the malformed leading-comma constraint should be handled, then add a regression test using this reproducer and verify that clang no longer asserts.

Written by the indexing model from the issue text.