[mips] The register names depend on the ABI being N32/N64 rather than the arch being mips64

Summary: Added test cases for O32 and N32 on MIPS64.

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D3175

llvm-svn: 204796