[mips][msa] Correct the behaviour of the COPY_FW pseudo on lanes 2 and 3.

Summary:
Previously, attempting to extract lanes 2 and 3 would actually extract lane 1.
The MSA CodeGen tests only covered lanes 0 and 1.

Differential Revision: http://llvm-reviews.chandlerc.com/D2935

llvm-svn: 202848