[mips] Add COP0 register class and use it in M[FT]C0/DM[FT]C0.

Summary:
Previously it (incorrectly) used GPR's.

Patch by Simon Dardis. A couple small corrections by myself.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10567

llvm-svn: 240883