[mips] Accept but ignore -m(no-)branch-likely

-mbranch-likely and -mno-branch-likely are used in some build systems for
some MIPS targets. Accept these options but ignore them as they are an
(de)optimiztion hint, and that branch likely instructions were deprecated
but not removed from MIPS32 and MIPS64 ISAs.

Reviewers: atanasyan, nitesh.jain

Differential Revision: https://reviews.llvm.org/D38168

llvm-svn: 314213