RFR: Make sure selective barriers enabling/disabling works

[Aleksey Shipilev]

On 11/22/2017 09:17 PM, Aleksey Shipilev wrote:
> 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}

Added a few checks for ShenandoahCloneBarrier in barrier set slowpaths:
  http://cr.openjdk.java.net/~shade/shenandoah/barrier-disable-asserts/webrev.02/

-Aleksey