RFR: 8238867: Improve G1DirtyCardQueueSet::Queue::pop

[Ivan Walulya]

This is a good fix to blocking on the last element. (Not a reviewer).

> On 12 Feb 2020, at 01:45, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
> Please review this change to G1DirtyCardQueueSet::Queue::pop.
> Previously, if there was exactly one element in the queue, a pop
> operation could not return it, because doing so could break invariants
> for concurrent operations.  Now, if there is one element and there are
> concurrent pop operations, one of those operations will win.  Note
> that there are still races between pop and push/append that may
> prevent the pop operation from obtaining an element.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8238867
> 
> Webrev:
> https://cr.openjdk.java.net/~kbarrett/8238867/open.00/
> 
> Testing:
> mach5 tier1-3.
> mach5 tier1-5 (only linux-x64) in conjunction with other changes.
> Some performance testing didn't find any unexpected differences.
>