[16] 8248041: Shenandoah: pre-Full GC root updates may miss some roots

[Zhengyu Gu]

Updating roots before full GC, should really belong to prepare phase.
The roots may contain forwarded pointers, only when this full GC is 
triggered by evacuation OOM, that leaves forwarded objects in roots.

Moving to prepare phase, while has_forwarded_objects flag still valid, 
also allows us to bypass this phase, if no forwarded objects in heap.


Bug: https://bugs.openjdk.java.net/browse/JDK-8248041
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8248041/webrev.00/

Test:
   hotspot_gc_shenandoah

Thanks,

-Zhengyu