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

[Roman Kennke]

Looks good

On Mon, Feb 12, 2018 at 3:33 PM, Aleksey Shipilev <shade at redhat.com> wrote:
> 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
>