RFR: Make sure selective barriers enabling/disabling works

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/barrier-disable-asserts/webrev.01/

This adds asserts and fixes up some of the barrier selection flags to make sure they work correctly.
All asserts are validated with new test that walks through interpreter, C1 and C2 with all
combinations of barrier flags. (It takes a while, around 400s for the test in fastdebug mode). This
code would be more interesting in sh/jdk8u, which lacks some of the flag guards.

This allows us to make sure our performance experiments around barriers are correct, as well as
check that "passive" mode indeed has no barriers enabled.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey