RFR: 8205923: ZGC: Verification applies load barriers before verification

[Stefan Karlsson]

Hi all,

Please review this patch to stop applying load barriers in the Mark End 
verification.

http://cr.openjdk.java.net/~stefank/8205923/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8205923

The recent IN_CONCURRENT_ROOT changes introduced load barriers to our 
Mark End verification. These load barriers are unnecessary because all 
roots should have been fixed at that point.

The object oop field verification applied a load barrier to be able to 
load stale j.l.Reference::referents, and as an effect applied load 
barriers to all fields. This patch skips visiting the referents and uses 
RawAccess loads for all other fields.

This patch builds upon the JDK-8205922, which introduces the mechanism 
to skip visiting referents in oop_iterate calls.

Thanks,
StefanK