[llvm] Fix for revert of dea5aa73f (PR #134153)

[Matt Arsenault via llvm-commits]

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@@ -127,6 +127,7 @@ class SIFixSGPRCopies {
   unsigned NextVGPRToSGPRCopyID = 0;
   MapVector<unsigned, V2SCopyInfo> V2SCopies;
   DenseMap<MachineInstr *, SetVector<unsigned>> SiblingPenalty;
+  SmallSet<MachineInstr *, 4> PHISources;
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arsenm wrote:

If this is a function wide set it's not going to be small 

https://github.com/llvm/llvm-project/pull/134153