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

[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