[llvm] [AMDGPU][NFCI] Decouple actual register encodings from HWEncoding values. (PR #69452)

[Ivan Kosarev via llvm-commits]

kosarev wrote:

As same questions about naming numbers keep coming up, I thought maybe proceeding by smaller steps will get us farther with this. I excluded changes related to `EncValues`. Please take another look.

https://github.com/llvm/llvm-project/pull/69452