[sh/8u] RFR 8229474: Shenandoah: Cleanup CM::update_roots()

[Zhengyu Gu]

I would like to backport this patch to shenandoah/8u.

The original patch does not apply cleanly, but conflicts are easy to 
resolve. Also, there is no traversal GC, that part of patch is no longer 
applicable.

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

sh/8u webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/JDK-8229474-8u/webrev.00/

Test:
   hotspot_gc_shenandoah


Thanks,

-Zhengyu