RFR: 8202081: Introduce CollectedHeap::is_oop()

[Stefan Karlsson]

Hi all,

I'd like to propose that we add a virtual CollectedHeap::is_oop() 
function, so that GCs can add GC specific checks when verifying oops.

We've been running with this patch in the ZGC repository for a while, 
but the patch is ZGC agnostic:
http://hg.openjdk.java.net/zgc/zgc/rev/66eb1ad3de72
https://bugs.openjdk.java.net/browse/JDK-8202081

(I can create a webrev if needed)

Thanks,
StefanK