[mips][msa] Added missing check in performSRACombine

Reviewers: jacksprat, dsanders

Reviewed By: dsanders

Differential Revision: http://llvm-reviews.chandlerc.com/D1755

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191495 91177308-0d34-0410-b5e6-96231b3b80d8