[mips] Add cache and pref instructions

Summary:
cache and pref were added in MIPS-III, and MIPS32 but were re-encoded in
MIPS32r6/MIPS64r6 to use a 9-bit offset rather than the 16-bit offset
available to earlier cores.

Resolved the decoding conflict between pref and lwc3.

Depends on D4115

Reviewers: zoran.jovanovic, jkolek, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D4116