[mips][msa] Fix element extraction where the index is variable.

Summary:
This isn't supported directly so we splat the vector element and extract
the most convenient copy.

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://reviews.llvm.org/D3530

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207524 91177308-0d34-0410-b5e6-96231b3b80d8