RFR(XXS) 8224495: Shenandoah: Do not rescan code roots in final mark pause if it is not degenerated GC

[Zhengyu Gu]

As comments in code stated, it only guards against degenerated GC, that 
bypasses concurrent cycle. So, let's only re-scan code roots if 
degenerated GC is in progress.

Bug: https://bugs.openjdk.java.net/browse/JDK-8224495
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8224495/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu