[crash, reproducer] X86 DAG->DAG Instruction Selection

# context

i'm working on https://github.com/attila-lendvai/maru and trying to access `argv` and `argc` on a freestanding build directly on top of the linux kernel.

with current trunk, i.e. beyond version 21.1.0

# reproducer

https://godbolt.org/z/n78dsvnPo

```llvm
%word = type i64
%oop = type ptr
declare ptr @llvm.sponentry()

define %oop @m__start.code() {
%r3 = call ptr @llvm.sponentry()
ret %oop %r3
}
```

output from the actual context:

```
clang -O3 -nostdlib -nostartfiles -ffreestanding -Wl,-Bstatic,-Ttext=0x08048000,-no-pie -Qunused-arguments --target=x86_64-linux-gnu -o build/llvm-linux/x86_64-linux-gnu/eval1 build/llvm-linux/x86_64-linux-gnu/eval1.ll
warning: overriding the module target triple with x86_64-unknown-linux-gnu [-Woverride-module]
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: /gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/bin/clang-20 -cc1 -triple x86_64-unknown-linux-gnu -emit-obj -dumpdir build/llvm-linux/x86_64-linux-gnu/eval1- -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name eval1.ll -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=none -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -ffreestanding -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/home/alendvai/common-lisp/maru -fcoverage-compilation-dir=/home/alendvai/common-lisp/maru -resource-dir /gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/lib/clang/20 -O3 -ferror-limit 19 -fmessage-length=159 -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcolor-diagnostics -vectorize-loops -vectorize-slp -faddrsig -o /tmp/eval1-cfad08.o -x ir build/llvm-linux/x86_64-linux-gnu/eval1.ll
1. Code generation
2. Running pass 'Function Pass Manager' on module 'build/llvm-linux/x86_64-linux-gnu/eval1.ll'.
3. Running pass 'X86 DAG->DAG Instruction Selection' on function '@m__start.code'
#0 0x00007f12adc293fb llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0xc293fb)
#1 0x00007f12adc26996 (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0xc26996)
#2 0x00007f12aca51b00 __restore_rt (/gnu/store/yj053cys0724p7vs9kir808x7fivz17m-glibc-2.41/lib/libc.so.6+0x3fb00)
#3 0x00007f12b24c9fc8 (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0x54c9fc8)
#4 0x00007f12ae6e7d1a (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0x16e7d1a)
#5 0x00007f12ae6eb235 llvm::SelectionDAG::Legalize() (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0x16eb235)
#6 0x00007f12ae8d0066 llvm::SelectionDAGISel::CodeGenAndEmitDAG() (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0x18d0066)
#7 0x00007f12ae8d2be4 llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0x18d2be4)
#8 0x00007f12ae8d4ac8 llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0x18d4ac8)
#9 0x00007f12ae8c2f29 llvm::SelectionDAGISelLegacy::runOnMachineFunction(llvm::MachineFunction&) (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0x18c2f29)
#10 0x00007f12ae1d7193 (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0x11d7193)
#11 0x00007f12addfcd89 llvm::FPPassManager::runOnFunction(llvm::Function&) (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0xdfcd89)
#12 0x00007f12addfcf2c llvm::FPPassManager::runOnModule(llvm::Module&) (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0xdfcf2c)
#13 0x00007f12addfc154 llvm::legacy::PassManagerImpl::run(llvm::Module&) (/gnu/store/rrrgwn9xnx5v4k4bmlkf499vi8d6p8cd-llvm-20.1.8/lib/libLLVM.so.20.1+0xdfc154)
#14 0x00007f12b8cd69db clang::emitBackendOutput(clang::CompilerInstance&, clang::CodeGenOptions&, llvm::StringRef, llvm::Module*, clang::BackendAction, llvm::IntrusiveRefCntPtr, std::unique_ptr>, clang::BackendConsumer*) (/gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/lib/libclang-cpp.so.20.1+0x24d69db)
#15 0x00007f12b922c6eb clang::CodeGenAction::ExecuteAction() (/gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/lib/libclang-cpp.so.20.1+0x2a2c6eb)
#16 0x00007f12b9cc9ad1 clang::FrontendAction::Execute() (/gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/lib/libclang-cpp.so.20.1+0x34c9ad1)
#17 0x00007f12b9c42dee clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/lib/libclang-cpp.so.20.1+0x3442dee)
#18 0x00007f12b9d68676 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/lib/libclang-cpp.so.20.1+0x3568676)
#19 0x00000000004161bf cc1_main(llvm::ArrayRef, char const*, void*) (/gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/bin/clang-20+0x4161bf)
#20 0x000000000040e7f3 (/gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/bin/clang-20+0x40e7f3)
#21 0x0000000000411f7d clang_main(int, char**, llvm::ToolContext const&) (/gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/bin/clang-20+0x411f7d)
#22 0x000000000040e203 main (/gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/bin/clang-20+0x40e203)
#23 0x00007f12aca3cbd7 __libc_start_call_main (/gnu/store/yj053cys0724p7vs9kir808x7fivz17m-glibc-2.41/lib/libc.so.6+0x2abd7)
#24 0x00007f12aca3cc95 __libc_start_main@GLIBC_2.2.5 (/gnu/store/yj053cys0724p7vs9kir808x7fivz17m-glibc-2.41/lib/libc.so.6+0x2ac95)
#25 0x000000000040e241 _start (/gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/bin/clang-20+0x40e241)
clang: error: unable to execute command: Segmentation fault
clang: error: clang frontend command failed due to signal (use -v to see invocation)
clang version 20.1.8
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /gnu/store/0nmbzahh7blsn8xp8kmasa6zy147k5cd-clang-20.1.8/bin
clang: note: diagnostic msg: Error generating preprocessed source(s) - no preprocessable inputs.
```

Start by running the minimal LLVM IR reproducer from the Godbolt link with clang -O3 and confirm the crash in the X86 DAG->DAG Instruction Selection pass. Trace the failing path in the SelectionDAG legalization or instruction-selection entry points; done means the reproducer no longer segfaults and the regression is covered by an appropriate test.

Written by the indexing model from the issue text.