[15] RFR 8245880: Shenandoah: check class unloading flag early in concurrent code root scan

[Zhengyu Gu]

Please review this minor improvement. If class unloading is enabled, 
there is no need to claim code cache.


Bug: https://bugs.openjdk.java.net/browse/JDK-8245880
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8245880/webrev.01/

Test:
   hotspot_gc_shenandoah


Thanks,

-Zhengyu