RFR: JDK-8224626: Shenandoah: Elide barriers on uncommon traps

[Roman Kennke]

 We currently preserve all load-reference-barriers that feed into calls.
With Aleksey's recent change JDK-8224522 that places barriers at deopt
unpacking, we don't need to place barriers in generated code when an oop
only feeds into uncommon traps.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8224626
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8224626/webrev.00/

Testing: hotspot_gc_shenandoah, specjbb, specjvm all good

Ok?

Roman