[llvm] [AMDGPU] NFC: Add helper to query archVGPR (PR #102690)

[Matt Arsenault via llvm-commits]

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@@ -54,6 +54,8 @@ struct GCNRegPressure {
     }
     return std::max(Value[VGPR32], Value[AGPR32]);
   }
+  unsigned getArchVGPRNum() const { return Value[VGPR32];}
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arsenm wrote:

Should get comments? Also could just fold this into the usage PR 

https://github.com/llvm/llvm-project/pull/102690