[mips] Add highly experimental support for MIPS-I, MIPS-II, MIPS-III, and MIPS-V

[mips] Add highly experimental support for MIPS-I, MIPS-II, MIPS-III, and MIPS-V

Summary:
These processors will only be available for the integrated assembler at
first (CodeGen will emit a fatal error saying they are not implemented).

The intention is to work through the existing instructions and correctly
annotate the ISA they were added in so that we have a sufficiently good
base to start MIPS64r6 development. MIPS64r6 removes/re-encodes certain
instructions and I believe it is best to define ISA's using set-union's
as far as possible rather than using set-subtraction.

Reviewers: vmedic

Subscribers: emaste, llvm-commits

Differential Revision: http://reviews.llvm.org/D3569

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208221 91177308-0d34-0410-b5e6-96231b3b80d8