RFR 8227635: Shenandoah: SHBSA::load_at() needs to deal IN_NATIVE load

[Zhengyu Gu]

Please review this patch that handles IN_NATIVE load in 
ShenandoahBarrierSetAssembler::load_at().


Bug: https://bugs.openjdk.java.net/browse/JDK-8227635
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8227635/webrev.00/index.html

Test:
   hotspot_gc_shenandoah (fastdebug and release) on
   Linux x64 and arrach64.

Thanks,

-Zhengyu