Enable optimization of arraycopy as loads/stores with Shenandoah

[Roland Westrelin]

> This removes the call to SharedRuntime::shenandoah_clone_barrier().

It doesn't remove it. It moves it around. It's now only added to those
clones that were not converted to loads/stores. Clone when it's not
converted to loads/stores uses bulk copies. So it's not obvious to me
that we can do better than using the
SharedRuntime::shenandoah_clone_barrier() call. I also added a test for
any object fields so the call to
SharedRuntime::shenandoah_clone_barrier() is not emitted when it's
obviously not needed.

Roland.