[mips][msa] Correct pattern for LSA

Summary:
$rs and $rt were the wrong way round in the .td and the testcase wasn't
strict enough to detect the mistake.

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D2554

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199498 91177308-0d34-0410-b5e6-96231b3b80d8