RFR: Support VerifyBeforeGC and VerifyAfterGC VM options

[Zhengyu Gu]

> 
> Yes. That's what exactly I were doing last past hour.
> 
> But why final-evac? I think should be final-mark.
> 
> Webrev: 
> http://cr.openjdk.java.net/~zgu/shenandoah/verify_before_after_gc/webrev.01/index.html 
> 
> 
Passed tier3_gc_shenandoah and  gc/TestVerifySubSet.java
(fastdebug and release)

Thanks,

-Zhengyu

> Test in progress.
> 
> Thanks,
> 
> -Zhengyu
> 
> 
> 
> 
> 
>>
>> You don't need new VMOps then.
>>
>> -Aleksey
>>