[llvm] [AMDGPU] Allocate AVRegClass last (PR #146606)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Previously, each RC with a distinct size, had a distinct priority. However, this PR groups together multiple sizes to the same priority. Currently, this will have no effect on prioritization in practice because we only have one actually defined RC per group per vector register type.

So why does this patch affect codegen for GFX11 and GFX12, which don't have AGPRs?

https://github.com/llvm/llvm-project/pull/146606