[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204796 91177308-0d34-0410-b5e6-96231b3b80d8