[mips] Add instruction alias (negu).

Summary: negu $reg is equivalent to negu $reg, $reg.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D3510

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207673 91177308-0d34-0410-b5e6-96231b3b80d8