[mips][msa] Fix suboptimal FrameIndex lowering for ld.[hwd] and st.[hwd]

Summary:
The immediate in these instructions is scaled before use as an offset.
They therefore have a wider reach than ld.b/st.b.

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D2338

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196775 91177308-0d34-0410-b5e6-96231b3b80d8