[MC] Check if register is non-null before calling isSubRegisterEq (NFCI)

D151036 adds an assertions that prohibits iterating over sub- and
super-registers of a null register. This is already the case when
iterating over register units of a null register, and worked by
accident for sub- and super-registers.

Reviewed By: Amir

Differential Revision: https://reviews.llvm.org/D151285