[llvm] Reland [SelectionDAG] Folding ZERO-EXTEND/SIGN_EXTEND poison to Poison value in getNode (PR #136701)

[via llvm-commits]

github-actions[bot] wrote:

<!--LLVM CODE FORMAT COMMENT: {undef deprecator}-->


:warning: undef deprecator found issues in your code. :warning:

<details>
<summary>
You can test this locally with the following command:
</summary>

``````````bash
git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 'HEAD~1' HEAD llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp llvm/test/CodeGen/AArch64/arm64-bitfield-extract.ll llvm/test/CodeGen/AArch64/optimize-cond-branch.ll llvm/test/CodeGen/AArch64/sve-extract-element.ll llvm/test/CodeGen/AMDGPU/amdgcn.bitcast.16bit.ll llvm/test/CodeGen/AMDGPU/atomic_optimizations_global_pointer.ll llvm/test/CodeGen/AMDGPU/ctpop16.ll llvm/test/CodeGen/AMDGPU/dead-machine-elim-after-dead-lane.ll llvm/test/CodeGen/AMDGPU/mdt-preserving-crash.ll llvm/test/CodeGen/PowerPC/undef-args.ll llvm/test/CodeGen/RISCV/miss-sp-restore-eh.ll llvm/test/CodeGen/VE/Vector/ticket-64420.ll llvm/test/CodeGen/X86/avx512-i1test.ll llvm/test/CodeGen/X86/bfloat.ll llvm/test/CodeGen/X86/clobber_frame_ptr.ll llvm/test/CodeGen/X86/concat-fpext-v2bf16.ll llvm/test/CodeGen/X86/jump_sign.ll llvm/test/CodeGen/X86/machine-trace-metrics-crash.ll llvm/test/CodeGen/X86/pr50254.ll llvm/test/CodeGen/X86/pr57673.ll llvm/test/CodeGen/X86/pr63108.ll llvm/test/CodeGen/X86/pr91005.ll llvm/test/CodeGen/X86/ragreedy-hoist-spill.ll llvm/test/CodeGen/X86/shift-combine.ll llvm/test/CodeGen/X86/shuffle-combine-crash.ll llvm/test/CodeGen/X86/shuffle-half.ll llvm/test/CodeGen/X86/swifterror.ll llvm/test/CodeGen/X86/tailcall-cgp-dup.ll llvm/test/CodeGen/X86/vaargs-prolog-insert.ll llvm/test/CodeGen/X86/vector-shuffle-combining-avx512bwvl.ll llvm/test/CodeGen/X86/x86-shrink-wrapping.ll
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</details>


The following files introduce new uses of undef:
 - llvm/test/CodeGen/PowerPC/undef-args.ll

[Undef](https://llvm.org/docs/LangRef.html#undefined-values) is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields `undef`. You should use `poison` values for placeholders instead.

In tests, avoid using `undef` and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:
```llvm
define void @fn() {
  ...
  br i1 undef, ...
}
```

Please use the following instead:
```llvm
define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}
```

Please refer to the [Undefined Behavior Manual](https://llvm.org/docs/UndefinedBehavior.html) for more information.



https://github.com/llvm/llvm-project/pull/136701