[mips] Add ISA_<name> adverbs and start using them instead of AdditionalPredicates overrides

Summary:
One small functional change. The recently added PAUSE instruction now has
the HasStdEnc predicate which was accidentally removed by a Requires<>.

Depends on D3640

Reviewers: vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3641