[llvm] [AMDGPU] When allocating VGPRs, VGPR spills are not part of the prologue (PR #109439)

[via llvm-commits]

alex-t wrote:

I have tested exactly the same change myself as an alternative to the https://github.com/llvm/llvm-project/pull/108596 
the change has passed PSDB but it only run on MI200. Unfortunately, it causes blender barbershop scene rendering to hang on Navi21. That is why I did not published the PR for it.
I am currently trying to find out the reason for the hang.

https://github.com/llvm/llvm-project/pull/109439