RFR: Resolve fwdptrs in CAS barrier without calling BarrierSet::resolve_for_read

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/cas-resolve-direct/webrev.01/

In CAS barrier, we are already at rock bottom barrier-generation-wise. So we should instead call our
own internal methods for resolving the fwdptrs on CAS fallback paths. Otherwise any change in
BarrierSet, including weird flag configuration, can screw up CAS barrier.

Testing: x86_64 (tier3_gc_shenandoah), aarch64 (cross-build)

Thanks,
-Aleksey