RFR: Missing cancelled concgc check results assertion failure

[Zhengyu Gu]

Diving to weak reference work without checking cancelled concgc, results 
assertion failure of not emptied task queues.

http://cr.openjdk.java.net/~zgu/shenandoah/tq_cancelled_gc/hs_err.txt

Webrev:
http://cr.openjdk.java.net/~zgu/shenandoah/tq_cancelled_gc/webrev.00/


Thanks,

-Zhengyu