[llvm] [AMDGPU] Fix RewriteMFMAFormSchedStage (PR #194887)

[Matt Arsenault via llvm-commits]

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@@ -2359,6 +2389,10 @@ int64_t RewriteMFMAFormStage::getRewriteCost(
   unsigned AGPRThreshold = MaxVectorRegs.second;
   unsigned CombinedThreshold = ST.getMaxNumVGPRs(MF);
 
+  // Reset the classes that were changed to AGPR for better RB analysis.
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arsenm wrote:

I'm guessing "RB" is "register bank", but should make this clearer 

https://github.com/llvm/llvm-project/pull/194887