RFR: [9] [backport] Make sure selective barriers enabling/disabling works

[Roman Kennke]

Am 27.11.2017 um 19:17 schrieb Aleksey Shipilev:
> http://cr.openjdk.java.net/~shade/shenandoah/barrier-disable-asserts/webrev.02.jdk9/
> 
> This is non-trivial backport to sh/jdk9. It is non-trivial, because barrier set is smaller. The
> accompanying test is supposed to verify the whole thing works as expected.
> 
> Testing: hotspot_gc_shenandoah {fastdebug|release}
> 
> Thanks,
> -Aleksey
> 

Looks good.