RFR: Cleanup and strengthen BrooksPointer verification

[Roman Kennke]

Am 12.02.2018 um 15:31 schrieb 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
>
>

Looks good