After insertion of (From, To), v is affected iff
depth(NCD)+1 < depth(v) && path P from To to v exists where every w on P s.t. depth(v) <= depth(w)
All affected vertices change their idom to NCD.
If a vertex u has changed its depth, it must be a descendant of an
affected vertex v. Its depth must have been updated by UpdateLevel()
called by setIDom() of the first affected ancestor.
So UpdateLevelsAfterInsertion and its bookkeeping variable VisitedNotAffectedQueue are redundant.
Run them only in debug builds as a sanity check.