RFR: Fix bad assertion: can see non-empty task queue during canceled GC cycle

[Aleksey Shipilev]

On 08/13/2018 07:09 PM, Zhengyu Gu wrote:
> An obvious bad assertion: non-empty task queues are possible if GC cycle is canceled.
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/bad_assertion/webrev.00/

Looks good. Please swap the sides before push: the actual rule should be first, and then any
exceptions from that rule to follow:

  assert(task_queues()->is_empty() || _heap->cancelled_gc(), "Should be empty");

Thanks,
-Aleksey