[llvm] [CodeGen] Introduce a RegisterUnit class to hold virtual reg or physical reg unit. NFC (PR #123768)

[Sergei Barannikov via llvm-commits]

https://github.com/s-barannikov commented:

When I tried to fix this, I ended up with a lot more changes. Does this patch deliberately not change `RegisterMaskPair` and its users to keep it small?


https://github.com/llvm/llvm-project/pull/123768