RFR 8233500: Shenandoah: Shenandoah load barrier should save registers before calling keep alive barrier on x86

[Zhengyu Gu]

This bug was found and fixed during concurrent class unloading work in 
shenandoah/jdk. However, I don't think it is concurrent class unloading 
specific issue, and could result hard to find problem in jdk/jdk.

BTW: AArch64 already does right thing.


Bug: https://bugs.openjdk.java.net/browse/JDK-8233500
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8233500/webrev.00/

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

Thanks,

-Zhengyu