[llvm] [InstCombine] Refactor fixed and scalable binop shuffle combine. NFCI (PR #141287)

[Luke Lau via llvm-commits]

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@@ -2094,6 +2094,50 @@ static bool shouldMergeGEPs(GEPOperator &GEP, GEPOperator &Src) {
   return true;
 }
 
+// Find constant NewC that has property:
+//   shuffle(NewC, ShMask) = C
+// Returns nullptr if such a constant does not exist e.g. ShMask=<0,0> C=<1,2>
+//
+// A 1-to-1 mapping is not required. Example:
+// ShMask = <1,1,2,2> and C = <5,5,6,6> --> NewC = <poison,5,6,poison>
+static Constant *unshuffleConstant(ArrayRef<int> ShMask, Constant *C,
+                                   VectorType *NewCTy) {
+  if (isa<ScalableVectorType>(NewCTy)) {
+    Constant *Splat = C->getSplatValue();
+    if (!Splat)
+      return nullptr;
+    return ConstantVector::getSplat(
+        cast<VectorType>(C->getType())->getElementCount(), Splat);
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lukel97 wrote:

Thanks for catching this, a fix and test should be added in dd1e8e8b0e29e8ad53a6a7675692241db224f02c

https://github.com/llvm/llvm-project/pull/141287