RFR(XS) 8223215: Shenandoah: Support verifying subset of roots

[Zhengyu Gu]

Please review this patch that allows to verify subset of roots.


Bug: https://bugs.openjdk.java.net/browse/JDK-8223215
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8223215/webrev.00/index.html

Test:
   hotspot_gc_shenandoah with -XX:+ShenandoahVerify (fastdebug and release)

Thanks,

-Zhengyu