[mips][mips64r6] Add experimental support for MIPS32r6 and MIPS64r6

Summary:
Adds MIPS32r6/MIPS64r6 and checks the compatibility requirements for these
processors.

I've also included comments to describe removed and re-encoded instructions,
along with placeholder def's for the new instructions but there are no
functional changes to codegen at this point.

Reviewers: jkolek, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3622