[mips] Only use FGR_{32,64} in TableGen descriptions. NFC.

Summary: Instead of explicitly adding the IsFP64bit and NotFP64bit predicates through AdditionalRequires.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9566

llvm-svn: 236835