RFR: ShenandoahVerifyOptoBarriers should not fail with disabled barriers

[Roman Kennke]

Am 04.12.2017 um 14:35 schrieb Aleksey Shipilev:
> http://cr.openjdk.java.net/~shade/shenandoah/c2-verify-barriers-disable/webrev.01/
> 
> This makes sure ShenandoahVerifyOptoBarriers does not fail with any combination of barriers. I tried
> to make it work with partially-enabled barriers, but it proves much more difficult that I
> anticipated. So, in this patch, we just disable verification if unusual barrier combination is
> requested. Also does some related code touchups.
> 
> Testing: hotspot_gc_shenandoah {fastdebug|release}
> 
> Thanks,
> -Aleksey
> 

Looks ok to me.

Roman