[mips] Replace FeatureFPIdx with FeatureMips4_32r2

Summary:
No functional change.

The minor change to the MIPS16 code is in preparation for a patch that will handle 32-bit FPIdx instructions separately to 64-bit (because they were added in different revisions)

Depends on D3677

Reviewers: rkotler, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3690