RFR: Remove BS:is_safe in favor of logged BS::verify_safe_oop

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/oop-is-safe/webrev.01/

We have introduced BarrierSet::is_safe to use in assertions, and we also have BS::verify_safe_oop
This seems excessive, and we instead would like to use verify_safe_oop on all paths, because it will
print out more sensible data on assertion failure. This also reduces our upstream exposure.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey