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

[Craig Topper via llvm-commits]

topperc wrote:

> 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?

Looks like RegisterMask is used by RegisterPressure.cpp/h which didn't intersect with any of the code I touched here.

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