RFR(XXS) 8224525: Shenandoah: Eliminate shenandoah verifier's side-effects

[Zhengyu Gu]

Since switching to LRB, verifier has side-effects of evacuating oops, if 
it is ran during evacuation phase.

The patch resets gc_state before LRB can be triggered by verifier, and 
restore gc_state when it is done.

Bug: https://bugs.openjdk.java.net/browse/JDK-8224525
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8224525/webrev.00/

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


Thanks,

-Zhengyu