[11u] RFR 8229474: Shenandoah: Cleanup CM::update_roots()

[Zhengyu Gu]

I would like to backport this patch to 11u, it is a Shenandoah-only patch.

The original patch does not apply cleanly, but conflicts are easy to 
resolve manually. Also, traversal GC part does not apply, as it has been 
removed.

The original bug: https://bugs.openjdk.java.net/browse/JDK-8229474
The original patch: https://hg.openjdk.java.net/jdk/jdk/rev/23e13076e102


11u webrev: http://cr.openjdk.java.net/~zgu/JDK-8229474-11u/webrev.00/

Test:
   hotspot_gc_shenandoah


Thanks,

-Zhengyu