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

[Roman Kennke]

>> Where exactly does verification interfere with evacuation-in-progress?
>> Because I don't see it. verify_before_evacuation() is done before
>> activating the flag, and verify_after_evacuation() is done after
>> clearing the flag.
> 
> There is also ShenandoahVerifier::verify_during_evacuation(). But anyhow, we want to skip any
> barriers when doing verification, as we'd like to see the unaided view of the heap.

Ok, that makes sense. Good with me.

Roman