[mips][msa] Matheus Almeida pointed out a silly mistake in r188893. Fixed it.

[mips][msa] Matheus Almeida pointed out a silly mistake in r188893. Fixed it.

I accidentally changed the encoding of the MSA registers to zero instead of 0
to 31. This change restores the encoding the registers had prior to r188893.

This didn't show up in the existing tests because direct-object emission isn't
implemented yet for MSA.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188896 91177308-0d34-0410-b5e6-96231b3b80d8