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

[Erik Österlund]

Hi,

The BarrierSet class has a bunch of unused or used in a seemingly 
nonsense way code. Some methods are used but only used in assertion, and 
are expected to return true (the has_*_opt methods). Others describe 
barriers that are never called from anywhere. This might have made sense 
at some point, but arguably does not any more. There is also an enum 
called Flags not referenced from anywhere.

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. It will also soon be superseded by the new GC barrier 
interface.

CR:
https://bugs.openjdk.java.net/browse/JDK-8189355

Webrev:
http://cr.openjdk.java.net/~eosterlund/8189355/webrev.00/

Thanks,
/Erik