[mips] NotMips64 predicate is really a test for 32-bit GPR's.

Summary:
Similarly, the HasMips64 on the 64-bit move InstAlias is a test for 64-bit
GPR's.

No functional change.

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://reviews.llvm.org/D3263