[llvm] [GlobalISel] Remove references to rhs of shufflevector if rhs is undef (PR #115076)

[Amara Emerson via llvm-commits]

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@@ -7721,3 +7721,32 @@ bool CombinerHelper::matchUnmergeValuesAnyExtBuildVector(const MachineInstr &MI,
 
   return false;
 }
+
+bool CombinerHelper::matchShuffleUndefRHS(MachineInstr &MI,
+                                          BuildFnTy &MatchInfo) {
+
+  bool Changed = false;
+  ArrayRef<int> OrigMask = MI.getOperand(3).getShuffleMask();
+  SmallVector<int, 8> NewMask;
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aemerson wrote:

16-wide byte vectors are common so let's bump this to 16.

https://github.com/llvm/llvm-project/pull/115076