RFR (L): 7173584: Implement arraycopy as a macro node

[Roland Westrelin]

> Also instead of duplicating gen_subtype_check() in macroArrayCopy.cpp can you modify the original one to work for you (by passing additional flag)?

This (on top of previous webrev - I've done only minimal testing)?
http://cr.openjdk.java.net/~roland/7173584/webrev.02/

Or do we want gvn_transform/gvn_transform_ctrl to be virtual methods of PhaseGVN/PhaseIterGVN?

Roland.