RFR: 8242597: Remove GenericTaskQueue<>::push_slow

[Ivan Walulya]

Looks good. 

> On 14 Apr 2020, at 05:10, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
> Please review this change which simplifies GenericTaskQueue::push() by
> removing some provably unnecessary code.  The case that push_slow was
> handling, where dirty_size == N-1, cannot occur.  Also added or
> updated some related comments.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8242597
> 
> Webrev:
> https://cr.openjdk.java.net/~kbarrett/8242597/open.00/
> 
> Testing:
> mach5 tier1-7
> 
>