[llvm] [AMDGPU] Constrain AV->VReg if we do not exceed RP thresholds (PR #150086)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> The issue is that the register class priority is dependent upon the register pressure situation. If we need to use AGPRs, then it is best to assign AV last, otherwise, it is best to assign based on the bitwidth of the register. To address minor increase in register usage caused by #146606

Can you just drop the AV priorities to the bottom regardless of bitwidth?

> 
> A more direct solution is to provide a hook at some level in DefaultPriorityAdvisor::getPriority to allow dynamic RC priorites based on RP situation.

I don't think the scheduler should be touching register classes at all. It has impacts outside of the current scheduling region, and the scheduler should not be increasing constraints 

https://github.com/llvm/llvm-project/pull/150086