RFR(M): 8210887: Tweak C2 gc api for arraycopy

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8210887/webrev.00/

This extends the entry point of the c2 access api for arraycopy (in
preparation for shenandoah). This also fixes some incorrect
_adr_type's. It also modifies the ArrayCopyNode::Ideal() transform that
produces a series of loads/stores so, as a subsequent change, barriers
can be added to loads and stores: they need to produce and consume
memory state other than the slice of the array being copied.

Roland.