[sema] Consolidate referencedActor->isSelf() checks in ActorReferenceResult::Builder::build into a single if block
The three consecutive checks that tested both referencedActor and
referencedActor->isSelf() are now nested under a single outer
if (referencedActor && referencedActor->isSelf()) guard, eliminating
the repeated condition. As a follow-on, the now-redundant isSelf() guard
at the top of checkedByFlowIsolation is replaced with an assert, since
all callers are required to have already established that precondition.