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

[Craig Topper via llvm-commits]

topperc wrote:

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

My understanding was that MCPhysReg was 16-bit primarily to reduce static data size in *GenRegisterInfo.inc. I'm probably the one who made it 16-bit. It doesn't provide any size savings in `RegisterMaskPair` because `LaneMask` has 32-bit alignment.

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

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