RFR: Cleanup and strengthen BrooksPointer verification

[Aleksey Shipilev]

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

This is the first salvo in verification refactoring: make sure we are verifying the cases where
fwdptr is NULL, and do not verify when fwdptr had failed to install with CAS. Also, protect from
accidental calls to verification in release bits.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey