[Mips] Fix missing masking in fast-isel of br (PR40325)

Fixes https://bugs.llvm.org/show_bug.cgi?id=40325 by zero extending
(and x, 1) the condition before branching on it.

To avoid regressing trivial cases, I'm combining emission of cmp+br
sequences for the single-use + same block case (similar to what we
do in x86). icmpbr1.ll still regresses due to the cross-bb usage
of the condition.

Differential Revision: https://reviews.llvm.org/D58576

llvm-svn: 354808