RFR 8232009: Shenandoah: C2 load barrier does not match interpreter version

[Zhengyu Gu]

Please review this patch that matches C2 load barrier to interpreter's 
implementation.

Bug: https://bugs.openjdk.java.net/browse/JDK-8232009
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8232009/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release) with x86_32 and x86_64 
JVMs on Linux


Thanks,

-Zhengyu