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

[Zhengyu Gu]

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