RFR: [8u] [backport] Make sure selective barriers enabling/disabling works

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/barrier-disable-asserts/webrev.03.jdk8u/

This is non-trivial backport to sh/jdk8u. It is non-trivial, because barrier set is smaller, *and*
many flags are missing. The accompanying test is supposed to verify the whole thing works as expected.

Testing: hotspot_gc_shenandoah {fastdebug|release}, benchmarks against Parallel

Thanks,
-Aleksey