RFR: ShenandoahVerifyOptoBarriers should not fail with disabled barriers

[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