We currently optimize the comparison only in SSA, therefore we will miss some optimization opportunities where the input of comparison is lowered from COPY in post-RA.
Ie. ExpandPostRA::LowerCopy is called after PPCInstrInfo::optimizeCompareInstr.
This patch optimizes the comparison in post-RA and only the cases that compare against zero can be handled.
D131374 converts the comparison and its user to a compare against zero with the appropriate predicate on the branch, which creates additional opportunities for this patch.