[mips] Added missing dsra -> dsrav and sra -> srav aliases.

Summary: dsll, dsrl, sll, and srl already exist.

Reviewers: vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3673

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208397 91177308-0d34-0410-b5e6-96231b3b80d8