RFR: [backport] 8231087: Shenandoah: Self-fixing load reference barriers for C1/C2

[Roman Kennke]

This backports:

https://bugs.openjdk.java.net/browse/JDK-8231087

to sh/jdk8. It accounts for lack of GC interfaces, but otherwise is 
relatively straightforward:

http://cr.openjdk.java.net/~rkennke/JDK-8231087-jdk8/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?

Roman