[mips] Implement jr.hb and jalr.hb (Jump Register and Jump and Link Register with Hazard Barrier).

Summary: These instructions are available in ISAs >= mips32/mips64. For mips32r6/mips64r6, jr.hb has a new encoding format.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D4019