[llvm] DAG: Use poison instead of undef in SimplifyDemandedVectorElts (PR #207788)

[Björn Pettersson via llvm-commits]

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@@ -3519,7 +3519,7 @@ bool TargetLowering::SimplifyDemandedVectorElts(
     // If none of the src operand elements are demanded, replace it with undef.
     if (!DemandedSrcElts && !Src.isUndef())
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bjope wrote:

Also turn UNDEF->POISON?

https://github.com/llvm/llvm-project/pull/207788