RFR(XS) 8235842: Shenandoah: Implement native LRB for narrow oop

[Roman Kennke]

Looks good. Go!

Thanks,
Roman

> Thanks Jie for reporting and verifying the problem.
> 
> Shenandoah used to assume that no compressed oops store in roots, but
> leak profiler store compressed oops in UnifiedOopRef.
> 
> This patch adds native LRB for compressed oop.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8235842
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8235842/webrev.00/
> 
> Test:
>   hotspot_gc_shenandoah (fastdebug and release)
>   Jie verified original bug with jmc.
> 
> Thanks,
> 
> -Zhengyu
>