RFR: rework CAS support

[Roland Westrelin]

> Excellent! This should reduce our upstream diff considerably. And the
> new stuff to support new C2 nodes should be ok too. Patch looks good,
> please push if it passes tier3_gc_shenandoah.

Except for:

#  Internal Error (/home/rwestrel/shenandoah/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp:80), pid=40463, tid=40517
#  Error: Shenandoah assert_not_forwarded failed; Object should not be forwarded

with AllocIntArrays, it does pass tier3_gc_shenandoah.

Roland.