RFR: JDK-8231086: Shenandoah: Stronger invariant for object-arraycopy

[mark.reinhold at oracle.com]

Perhaps you’re aware of this already, but after this change and its
predecessor (8231085) it takes significantly longer to compile HotSpot,
I’m guessing due to C++ template expansion.  I saw build times on a
four-core, eight-thread, 64GB Xeon box go from about 10m up to 15m.

(Yes, I’m aware that one could avoid this slowdown by not building
 Shenandoah, but who would want to do that?)

Bisecting the repo to identify this slowdown was tricky since the
changeset for 8231085 (b1a394e15ae9) leaves the repo in a state that
doesn’t build.  Please avoid that going forward -- every changeset
should leave the repo in a buildable state.

- Mark