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

[Roman Kennke]

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