[NFC] Let (MC)Register APIs check isStackSlot

The user is expected to make the isStackSlot check before calling isPhysicalRegister
or isVirtualRegister. The APIs assert otherwise. We can improve the usability
of these APIs by carrying out the check in the 2 APIs: they become a
complete "source of truth" and remove an extra responsibility from the
user.

Differential Revision: https://reviews.llvm.org/D88598