RFR(XS): 8039089 - List verification enabled in product builds

[Jesper Wilhelmsson]

Hi,

This is a one-liner to stop ConcurrentMark::completeCleanup() from running list 
verification in product builds.

It is currently calling _cleanup_list.verify_list(). This will unconditionally 
run the verification, even in product builds. I replaced it with 
verify_optional() that only runs the verification code in debug builds.

Bug: https://bugs.openjdk.java.net/browse/JDK-8039089

Webrev: http://cr.openjdk.java.net/~jwilhelm/8039089/webrev/

Thanks,
/Jesper