[NFC][MC] Type uses of MCRegUnitIterator as MCRegister

This is one of many subsequent similar changes. Note that we're ok with
the parameter being typed as MCPhysReg, as MCPhysReg -> MCRegister is a
correct conversion; Register -> MCRegister assumes the former is indeed
physical, so we stop relying on the implicit conversion and use the
explicit, value-asserting asMCReg().

Differential Revision: https://reviews.llvm.org/D88862