[llvm] [AMDGPU] Only legalize PHIs in SIFixSGPRCopies that need it (PR #226213)

[Pablo Reble via llvm-commits]

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@@ -7765,6 +7765,17 @@ SIInstrInfo::legalizeOperands(MachineInstr &MI,
       if (!Op.isReg() || !Op.getReg().isVirtual())
         continue;
 
+      // No copy is needed if the classes differ only in AV-ness, e.g. an av_32
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reble wrote:

Rework done in 10cd916. I dropped the special case in `legalizeOperands`;
`processPHINode` now only legalizes PHIs that need it, so PHIs that are VALU from the start are left alone.

I also tested with CK on gfx90a (291 problems, ~26k instance runs): no new failures, and best-instance time is neutral (geomean 1.000). Spills in max-pressure MFMA kernels move both ways, net down (−3.6k).

https://github.com/llvm/llvm-project/pull/226213