[mips] Add FGR_32/FGR_64/GPR_64 adjectives and use then instead of FGRPredicates/GPRPredicates

Summary:
No functional change (confirmed by diffing tablegen-erated files).

Depends on D3642

Reviewers: vmedic, dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D3645