[llvm] [WIP][AMDGPU] Improve the handling of `inreg` arguments (PR #133614)

[Shilei Tian via llvm-commits]

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@@ -2841,6 +2841,86 @@ void SITargetLowering::insertCopiesSplitCSR(
   }
 }
 
+class InregVPGRSpiller {
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shiltian wrote:

Need document here.

https://github.com/llvm/llvm-project/pull/133614