Re: RFR(S) 228777: Shenandoah: Cleanup STW weak root processing

I am not sure about this. If final-mark could not finish to pre-evacuate all roots we might have from-space refs there. Or does the OOM-protocol ensure that any ref should be updated correctly? We need to be careful here. Better safe than sorry. Roman

Shenandoah pre-evacuates or updates weak roots at STW pauses. Therefore, weak roots should not have forwarded references during mark phase.

Current has_forwarded_objects() branch does not break things, but does unncessary works, let's prune it.

Bug: https://bugs.openjdk.java.net/browse/JDK-8228777 Webrev: http://cr.openjdk.java.net/~zgu/JDK-8228777/webrev.00/

Test:   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu