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

[Björn Pettersson via llvm-commits]

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@@ -3441,7 +3441,7 @@ bool TargetLowering::SimplifyDemandedVectorElts(
         bool Updated = false;
         for (unsigned i = 0; i != NumElts; ++i) {
           if (!DemandedElts[i] && !Ops[i].isUndef()) {
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bjope wrote:

Also turn UNDEF->POISON?

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