RFR (S) 8248652: Shenandoah: SATB buffer handling may assume no forwarded objects

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8248652

Since CM-with-UR is gone, SATB may assume no forwarded objects are ever exposed through it. The only
way marking code can experience forwarded objects is due to Full GC marking. In that case, SATB
should be inactive.

Fix:
  https://cr.openjdk.java.net/~shade/8248652/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey