[llvm] [DAG] narrowExtractedVectorBinOp - ensure we limit late node creation to LegalOperations only (PR #72130)

[Simon Pilgrim via llvm-commits]

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@@ -270,10 +270,11 @@ define i64 @kshiftl_v64i1_63(<64 x i8> %x, <64 x i8> %y) {
 ; KNL-NEXT:    vpcmpeqb %xmm2, %xmm0, %xmm0
 ; KNL-NEXT:    vpmovsxbd %xmm0, %zmm0
 ; KNL-NEXT:    vptestmd %zmm0, %zmm0, %k0
+; KNL-NEXT:    vpxor %xmm0, %xmm0, %xmm0
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RKSimon wrote:

This is an known existing (very old) issue we're having in that the 128/256/512-bit zero vectors don't get merged properly: #26392

https://github.com/llvm/llvm-project/pull/72130