RFR: Verify fwdptr accesses during Full GC moves

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/verify-fullgc-moves/webrev.01/

This is the follow-up for the failures we have seen today around C1 compiler bugs. Verifier should
capture them more consistently by verifying that no fwdptr access is observing the bad pointers
during Full GC moves.

Testing: hotspot_gc_shenandoah, + reverting the compiler workaround and observing the failure

Thanks,
-Aleksey