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

[Zhengyu Gu]

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