[mips][msa] Fix vector insertions where the index is variable

Summary:
This isn't supported directly so we rotate the vector by the desired number of
elements, insert to element zero, then rotate back.

The i64 case generates rather poor code on MIPS32. There is an obvious
optimisation to be made in future (do both insert.w's inside a shared 
rotate/unrotate sequence) but for now it's sufficient to select valid code
instead of aborting.

Depends on D3536

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://reviews.llvm.org/D3537

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207640 91177308-0d34-0410-b5e6-96231b3b80d8