RFR 8226586: Shenandoah: No need to pre-evacuate roots for degenerated G

[Roman Kennke]

Looks good. Thanks!

Roman


> There is no need to pre-evacuate roots during degenerated GC. The roots
> are updated after evacuation in final update references phase.
> 
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8226586
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8226586/webrev.00/
> 
> Test:
>   hotspot_gc_shenandoah (fastdebug and release)
> 
> Thanks,
> 
> -Zhengyu