[mips] Remove codegen support for branch likely instructions.

This patch disables codegen support for branch likely instructions to
address a potential bug. These branches were unselectable as
they had the same patterns as the normal branches but came after them
when ISel was concerned.

The branch likely instructions were marked as having no delay
slots when they have annulling delay slots. The delay slot filler
does not currently handle annulling delay slot branches, so this
would lead to wrong codegen if these branches were generated.

Reviewers: atanasyan, nitesh.jain

Differential Revision: https://reviews.llvm.org/D38169

llvm-svn: 314421