[mips] Prevent %lo relocation being used on MSA loads and stores.

Summary:
Parts of the compiler still believed MSA load/stores have a 16-bit offset when
it is actually 10-bit. Corrected this, and fixed a closely related issue this
uncovered where load/stores with 10-bit and 12-bit offsets (MSA and microMIPS
respectively) could not load/store using offsets from the stack/frame pointer.
They accepted frameindex+offset, but not frameindex by itself.

Reviewers: jacksprat, matheusalmeida

Reviewed By: jacksprat

Differential Revision: http://llvm-reviews.chandlerc.com/D2888

llvm-svn: 202717