RFR (S): 8189355: Cleanup of BarrierSet barrier functions

[Kim Barrett]

> On Oct 17, 2017, at 6:27 AM, Erik Österlund <erik.osterlund at oracle.com> wrote:

Not a review yet; still looking at it.

> There is also an enum called Flags not referenced from anywhere.

See https://bugs.openjdk.java.net/browse/JDK-8072956.
I’m okay with removing it.

> I would also like to get rid of devirtualize_reference_writes() which devirtualizes calls to the post-write barrier for card marking specifically by loading and comparing the type of barrier set. I have run a bunch of benchmarks and this optimization did not seem to provide any benefit.

Yay!