[llvm] [AMDGPU] Handle AV classes in SIFixSGPRCopies::processPHINode (PR #169038)

[Jay Foad via llvm-commits]

jayfoad wrote:

As I said in the bug report:

> `isVectorRegister` seems pernicious because it only accepts registers of VGPR-only or AGPR-only classes, not the combined AV classes.

So maybe we should eliminate all uses of `isVectorRegister`. I can't think of any good uses for it.

https://github.com/llvm/llvm-project/pull/169038