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

[Zhengyu Gu]

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/

Test:

   tier2_gc_shenandoah (fastdebug) and Compiler.sunflow where the 
failure was seen.

Thanks,

-Zhengyu