[14] RFR 8240873: Shenandoah: Short-cut arraycopy barriers

[Aleksey Shipilev]

Original RFE:
  https://bugs.openjdk.java.net/browse/JDK-8240873
  https://hg.openjdk.java.net/jdk/jdk/rev/cc0ffb1d0458

The context is different, because 15 did HeapWord* -> oop conversions on many paths. The whole thing
would be followed by other changes that drop volatiles/atomics from update_watermark.

14u webrev:
  https://cr.openjdk.java.net/~shade/8240873/webrev.14u.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey