[15] RFR 8245827: Shenandoah: Cleanup Shenandoah code root iterators and root scanner

[Zhengyu Gu]

Hi Aleksey,

>> Bug: https://bugs.openjdk.java.net/browse/JDK-8245827
>> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8245827/webrev.00/index.html
> 
> Looks good!

Thanks.

> 
> For the code that is moved around, there were no actual semantic changes, right? I haven't checked
> that part closely.
> 

Other than moving code around, also de-templatized 
ShenandoahCodeRootIterator and ShenandoahRootScanner (which were the 
reasons to move code around).

Okay?

Thanks,

-Zhengyu