RFR 8231324: Shenandoah: avoid duplicated weak root works during final traversal

[Zhengyu Gu]

Please review this patch that avoids traversal GC to walk weak roots 
twice during final traversal.

Also, it should process weak roots first, so that, fixup phase does not 
visit dead CLDs/codes, etc.

Bug: https://bugs.openjdk.java.net/browse/JDK-8231324
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8231324/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release) on Linux x86_64

Thanks,

-Zhengyu