[CFG] Add shortcut if CycleInfo is available (#188928)
isPotentiallyReachable() currently returns "reachable" early if BB
dominates StopBB. If CycleInfo is available, and BB is not part of a
cycle, we can also perform the reverse inference: Return "not reachable"
if StopBB dominates BB.
This both allows aborting the walk earlier, and provides a more precise
result.
