RFR: 8256401: ZGC: Improve ZList verification [v3]

[Per Liden]

On Tue, 17 Nov 2020 07:00:11 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Per Liden has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Review 1
>
> Marked as reviewed by stefank (Reviewer).

Thanks for reviewing!

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PR: https://git.openjdk.java.net/jdk/pull/1230