RFR (S): [8] Cherry-pick fix for "8175887: C1 value numbering handling of Unsafe.get*Volatile is incorrect"

[Aleksey Shipilev]

This allows shenandoah-8u to pass new jcstress.

Let's cherry-pick into shenandoah-jdk8u, before it proliferates via 8u152. We
did this exercise in shenandoah-jdk9 already, and we got the actual changeset
via the merge there.

https://bugs.openjdk.java.net/browse/JDK-8176451
http://hg.openjdk.java.net/jdk8u/jdk8u-dev/hotspot/rev/0e53cdb9fc2a

Thanks,
-Aleksey