[llvm] [SelectionDAG] Fold undemanded operand to UNDEF for VECTOR_SHUFFLE (PR #145524)

[Simon Pilgrim via llvm-commits]

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@@ -3587,6 +3587,19 @@ bool TargetLowering::SimplifyDemandedVectorElts(
         DemandedRHS.setBit(M - NumElts);
     }
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RKSimon wrote:

(side note) we have llvm::getShuffleDemandedElts to do this for us now

https://github.com/llvm/llvm-project/pull/145524