RFR (S) 8224751: Shenandoah: Shenandoah Verifier should select proper roots according to current GC cycle

[Roman Kennke]

Looks good. Thanks!

Roman


> Verifier should seed the marking with the same roots as current GC cycle.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8224751
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8224751/webrev.00/
> 
> Test:
>   hotspot_gc_shenandoah with ShenandoahVerify (fastdebug and release)
> 
> Thanks,
> 
> -Zhengyu
>