RFR: Support VerifyBeforeGC and VerifyAfterGC VM options

[Zhengyu Gu]

Adding VerifyBeforeGC and VerifyAfterGC, that gives Shenandoah to pass 
gc/TestVerifySubSet.java test.


Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/verify_before_after_gc/webrev.00/index.html

Test:

   tier3_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu