8202377: Modularize C2 GC barriers

[Roland Westrelin]

Hi Erik,

> http://cr.openjdk.java.net/~eosterlund/8202377/webrev.01/

Overall, this looks good to me.

I suppose:

loop_optimize_gc_barrier()
find_dominating_barriers()

are there for ZGC and they don't feel generic enough to me to be
included in this API. That is if I were to add another GC or let say
change the G1 implementation so it's better suited for optimizations, I
think it's quite unlikely I would reuse either hook. It would be better
to remove them in my opinion.

Roland.