[mips] Enforce compact branch register restrictions

Enforce compact branch register restrictions such as the use of the zero
register, both operands being the same register. Emit clear error in such
cases as the issue is subtle.

For bovc and bnvc, silently fixup such cases when emitting objects directly,
like LLVM started doing in rL269899.

Reviewers: vkalintiris, dsanders

Differential Review: http://reviews.llvm.org/D20475

llvm-svn: 271301