RFR 8225573: Shenandoah: Enhance ShenandoahVerifier to ensure roots to-space invariant

[Zhengyu Gu]

This patch adds a new API to Shenandoah verifier, the verifies all roots 
only contain to-space references. This API should help concurrent root 
processing work, to catch bad references in roots.

Bug: https://bugs.openjdk.java.net/browse/JDK-8225573
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8225573/webrev.00/8225573

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

Thanks,

-Zhengyu