RFR: Code roots scanning might be bypassed during degenerated cycle

[Zhengyu Gu]

Degenerated cycle might skip concurrent cycle and dive directly into 
final mark. When ShenandoahConcurrentScanCodeRoots is set, that means we 
may miss the scanning of code roots.

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


Thanks,

-Zhengyu