[llvm] [AMDGPU] Use correct VGPR threshold for flagging ExcessRP regions in unified register file case (PR #85860)

[Matt Arsenault via llvm-commits]

================
@@ -124,7 +124,7 @@ bool GCNRegPressure::less(const MachineFunction &MF, const GCNRegPressure &O,
   unsigned OtherVGPRForSGPRSpills =
       (OtherExcessSGPR + (WaveSize - 1)) / WaveSize;
 
-  unsigned MaxArchVGPRs = ST.getAddressableNumArchVGPRs();
+  unsigned MaxArchVGPRs = ST.getMaxNumVGPRs(MF, /*WholeRegisterFile*/ false);
----------------
arsenm wrote:

```suggestion
  unsigned MaxArchVGPRs = ST.getMaxNumVGPRs(MF, /*WholeRegisterFile=*/ false);
```

https://github.com/llvm/llvm-project/pull/85860