RFR: JDK-8220780: ShenandoahBS::AccessBarrier::oop_store_in_heap_at ignores AS_NO_KEEPALIVE

[Roman Kennke]

Aleksey discovered that we're ignoring AS_NO_KEEPALIVE for oop-stores.
This is bad because it means we do extra processing for no good reason
(enqueueing stuff in SATB) and worst case, keep stuff alive that we
shouldn't.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8220780
Cure:
http://cr.openjdk.java.net/~rkennke/JDK-8220780/webrev.00/

Passes hotspot_gc_shenandoah

Good to push?

Roman