RFR: 8227085: ZGC: Add on_weak load barrier verification

[Stefan Karlsson]

Hi all,

Please review this patch to add some verification to our on_weak load 
barriers.

http://cr.openjdk.java.net/~stefank/8227085/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8227085

We've had a few bugs where on_weak load barriers have been generated 
instead of the normal, on_strong, load barriers. The on_weak barriers 
should only be used to read the referent fields of WeakReferences and 
SoftReferences. The given oop* is used to calculate the address of the 
containing object and check that it is a valid Reference object.

This code was tested against the bugs in JDK-8225770 and JDK-8227083.

Tested with tier 1-7

Thanks,
StefanK