[mips] Correct the instruction predicates for microMIPSr3

Rather than using the AdditionalPredicates mechanism to guard
the microMIPS instructions, use the existing predicates to properly
guard those instructions.

This also resolves a case where an instruction pattern was incorrectly
available for microMIPS32R6, which caused a register allocation failure
as the registers specified in the pattern were not available.

Reviewers: nitesh.jain, atanasyan

Differential Revision: https://reviews.llvm.org/D38451

llvm-svn: 315362