RFR (S) 8222130: Shenandoah should verify roots after pre-evacuation

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8222130

Shenandoah verifier usually does the checks when the pause is over, and everything is done. One
notable exception is evacuation path, where we do the verify_before_evacuation() check, then
pre-evac the roots, and unblock. We need to check those roots are still fine after pre-evac by doing
another verification step.

Fix:
  http://cr.openjdk.java.net/~shade/8222130/wevrev.01/

Testing: Linux x86_64 fastdebug hotspot_gc_shenandoah

-Aleksey

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