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

[Roman Kennke]

Looks good to me, thank you!

Roman


> 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
>