[15] RFR(XS) 8236681: Shenandoah: Disable concurrent class unloading flag if no class unloading for the GC cycle

[Zhengyu Gu]

Please review this small patch that disables concurrent class unloading 
if there is no class unloading for the particular GC cycle.

This is not a fatal error, but can confuse verifier.


Bug: https://bugs.openjdk.java.net/browse/JDK-8236681
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8236681/webrev.00/
Test:
   hotspot_gc_shenandoah (fastdebug and release)


Thanks,

-Zhengyu