[16] RFR 8251359: Shenandoah: filter null oops before calling enqueue/SATB barrier

[Zhengyu Gu]

Please review this small patch that ensures null oops are filtered out 
before calling enqueue/SATB barrier. Enqueuing null oops create extra 
works and only get thrown out in later phase.


Bug: https://bugs.openjdk.java.net/browse/JDK-8251359
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8251359/webrev.00/index.html

Test:
   hotspot_gc_shenandoah

Thanks,

-Zhengyu