RFR: 8189871: Refactor GC barriers to use declarative semantics

[Kim Barrett]

> On Nov 21, 2017, at 2:16 AM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
>> On Nov 21, 2017, at 2:07 AM, Kim Barrett <kim.barrett at oracle.com> wrote:
>> 
>> Noticed another issue while looking for something else.
> 
> And another one.

Oh, and I’ve now noticed that this change has already been pushed.  I guess these
late comments should be turned into bug reports.