8238691: C2: turn subtype check into macro node

[Roland Westrelin]

> I was suggesting this because I incorrectly assumed that the nodes are always dead - which is not
> the case. Your version is better then but you might want to use PhaseGVN::record_for_igvn to avoid
> the is_IterGVN() check (no new webrev required).

record_for_igvn doesn't do anything on an PhaseIterGVN:

 virtual void record_for_igvn(Node *n) { }

Roland.