[mips][msa] Fix invalid generated code when lowering FrameIndex involving unaligned offsets.

Summary:
The MSA ld.[bhwd] and st.[bhwd] instructions scale the immediate by the
element size before use as an offset. The offset must therefore be a
multiple of the element size to be valid in these instructions. However,
an unaligned base address is valid in MSA.

This commit causes the compiler to emit valid code when the calculated
offset is not a multiple of the element size by accounting for the offset
using addiu and using a zero offset in the load/store.

Depends on D2338

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D2339