[llvm] [CodeGen] Rename RegisterMaskPair to RegUnitMaskPair. NFC (PR #123799)

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

> I'm open to other name suggestions.

(just to suggest something)
RegLanes? Or maybe RegLiveLanes, assuming it is not used for marking dead parts of a register.


https://github.com/llvm/llvm-project/pull/123799