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

[Stanislav Mekhanoshin via llvm-commits]

rampitec wrote:

This was done conservatively on purpose. That is true we have total of 512 VGPRs, but only half of these a general purpose. Doubling it will result in a higher register pressure on some programs and spilling because most of the instructions may not use AGPRs. I.e. you may have improvements in some cases you are working on, but there will be bigger regressions in some other cases.

If you'd like to do it, I would at the very least guard this accounting with a call to `MFI::usesAGPRs()`.

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