RFR: 8205339: ZGC: VerifyBeforeIteration not yet supported

[Per Liden]

Always disable the diagnostic flag VerifyBeforeIteration. This option is 
not yet supported for the same reason ZGC don't yet support VerifyStack, 
i.e. this code makes assumptions about the state of oops, which aren't 
true in a ZGC context.

Bug: https://bugs.openjdk.java.net/browse/JDK-8205339
Webrev: http://cr.openjdk.java.net/~pliden/8205339/webrev.0

/Per