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

[Zhengyu Gu]

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