[aarch64-port-dev ] RFR 8233401: Shenandoah: Refactor/cleanup Shenandoah load barrier code

[Zhengyu Gu]

Please review this refactor of Shenandoah load barrier. The goal is to 
make the barrier structurally similar cross interpreter, C1 and C2, 
improve readability and maintainability.

Bug: https://bugs.openjdk.java.net/browse/JDK-8233401
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8233401/webrev.00/index.html

Test:
   hotspot_gc_shenandoah (fastdebug and release)
   x86_64 and x86_32 on Linux
   AArch64 on Linux

Thanks,

-Zhengyu