RFR: (Re-)Enable arraycopy C2 optimizations for non-traversal GC

[Roman Kennke]

Looks like I've accidentally disabled the arraycopy-to-loadstores
optimization wholesale during the C2 barriers merge. We actually support
that optimization for non-traversal-GC. Traversal GC requires more
complicated storeval-barriers, that will probably come when Roland's
work arrives.

http://cr.openjdk.java.net/~rkennke/enable-arycopy-c2-opt/webrev.00/

Testing: tier3_gc_shenandoah (ongoing)

Ok?

Roman