RFR (XS): 8211853: Avoid additional duplicate work when a reference in the task queue has already been evacuated

[Kim Barrett]

> On Oct 16, 2018, at 9:59 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> New webrev at 
> http://cr.openjdk.java.net/~tschatzl/8211853/webrev.0_to_1 (diff)
> http://cr.openjdk.java.net/~tschatzl/8211853/webrev.1 (full)
> with the added assert.
> 
> Passed hs-tier1-3 as expected.
> 
> Thanks,
>  Thomas

Looks good.