[mips][msa] Test basic operations for the N32 ABI too.

Summary:
This required adding instruction aliases for dneg.

N64 will be enabled shortly but requires additional bugfixes.

Reviewers: vkalintiris

Reviewed By: vkalintiris

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9341

llvm-svn: 236489