RFR (S): 8209843: Optimize oop scan closure closures wrt to reference processing in G1

[Kim Barrett]

> On Sep 4, 2018, at 6:35 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> http://cr.openjdk.java.net/~tschatzl/8209843/webrev.1/ (full only,
> sorry, but the change is very small)
> 
> Thanks,
>  Thomas

I feel like there's a bit of a tangled mess in this area, with various
potential inconsistencies that make the code harder to understand than
it needs to be.  That said, I don't think your change is making that
any worse, and in some cases is making it better.  So okay, looks good.