RFR: Merge jdk/jdk -> shenandoah/jdk to jdk-13+13

[Roman Kennke]

The merge was mostly trivial, except for:

8220780: ShenandoahBS::AccessBarrier::oop_store_in_heap ignores
AS_NO_KEEPALIVE

which needed to move around a little bit due to cleaned-up ShBS.

Full list of changes:
http://cr.openjdk.java.net/~rkennke/upstream-jdk13-merge-2019-03-22/changes.txt

Testing: hotspot_gc_shenandoah good

OK?

Roman