[llvm] AMDGPU: Add amdgpu-num-agpr attribute to control AGPR allocation (PR #128034)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I found these ranges are very confusing. If I require minimum 0 AGPRs, that means I can require 1, 2, 3, etc. AGPRs, because they are all larger than the minimum. Why does it mean no AGPR?

The name is confusing, particularly since it's not the same behavior as amdgpu-num-vgpr and amdgpu-num-sgpr (which I would really like to delete). It probably should be changed.

The minimum is supposed to be the minimum possible requirement. If the minimum is 0, you cannot allocate any AGPRs.

https://github.com/llvm/llvm-project/pull/128034