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

[Zhengyu Gu]

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