[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);
     }
 
+    // If either side isn't demanded, replace it by UNDEF. We handle this
+    // explicitly here to also simplify in case of mulitple uses (on the
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RKSimon wrote:

mulitple -> multiple

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