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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> The `MachineBasicBlock::addLiveIn` function that creates `RegisterMaskPair` uses MCRegister in its interface. Are you suggesting to change that interface?

We should change it, but not in this way. We should stop tracking liveins by registers and use regunits instead. As it is now it doesn't matter which type it uses 

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