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

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/7173584/webrev.00/

When arraycopy is intrinsified, it is converted as calls to specialized stubs based on what's known from the context of the arraycopy. Right now, this is done at parsing. By using a new macro node for arraycopy that is expanded later, the compiler may be able to gather more data on the context of the arraycopy and make a better choice for the stub to call.

This also opens the door to other optimizations such as inlining small copies in a series of loads/stores.

This also impacts clone and Arrays.copyOf's implementations because they rely on the arraycopy stubs as well.

Roland.