Request for reviews (M): 6677629: PhaseIterGVN::subsume_node() should call hash_delete() and add_users_to_worklist()

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/6677629/webrev

Fixed 6677629: PhaseIterGVN::subsume_node() should call hash_delete() and add_users_to_worklist()

Currently C2 does not check that users of a subsumed node
are placed on IGVN worklist. Because of that some expressions
are not collapsed as expected.

I suggest to use replace_node() method instead of subsume_node().
Make subsume_node() private and use it only in transform_old().

Passed CTW, JPRT.