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

[Craig Topper via llvm-commits]

topperc 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

Ok so what should I do for this patch? I'm trying to fix the implicit conversion from MCRegister to MCPhysReg when the RegisterMaskPair constructor is used. Should I just add a .id() call instead?

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