[15] RFR 8240917: Shenandoah: Avoid scanning thread code roots twice in all root scanner

[Zhengyu Gu]

Please review this small enhancement, that avoids scanning thread's code 
roots if we scan all code blobs anyway.

Bug: https://bugs.openjdk.java.net/browse/JDK-8240917
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8240917/webrev.00/


Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu