[mips] Fix compact branch hazard detection

In certain cases it is possible that transient instructions such as
%reg = IMPLICIT_DEF as a single instruction in a basic block to reach
the MipsHazardSchedule pass. This patch teaches MipsHazardSchedule to
properly look through such cases.

Reviewers: vkalintiris, zoran.jovanovic

Differential Revision: https://reviews.llvm.org/D27209

llvm-svn: 289529