[llvm] [CodeGen] Use MCRegister instead of MCPhysReg in RegisterMaskPair. NFC (PR #123688)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Most of these are really MCPhysReg. There is a slight difference since MCPhysReg is 16-bit and MCRegister is 32 

https://github.com/llvm/llvm-project/pull/123688