[MIPS] Remove an incorrect microMIPS instruction alias

The microMIPS instruction set is compatible with the MIPS instruction
set at the assembly level but not in terms of encodings. `nop` in
microMIPS is a special case as it has the same encoding as `nop` for
MIPS.

Fix this error by reducing the usage of NOP in the MIPS backend such
that only that ISA correct variants are produced.

Differential Revision: https://reviews.llvm.org/D124716