[11u] RFR (S) 8259849: Shenandoah: Rename store-val to IU-barrier

[Aleksey Shipilev]

Original cleanup:
   https://bugs.openjdk.java.net/browse/JDK-8259849
   https://git.openjdk.java.net/jdk/commit/e60c9926

I had to basically redo most of the patch, because the context is different in many places. But the 
change is mechanical search-and-replace.

11u variant:
   https://cr.openjdk.java.net/~shade/8259849/webrev.11u.01/

Testing: hotspot_gc_shenandoah on Linux x86_64, x86_32, AArch64; also tier{1,2,3} with Shenandoah

-- 
Thanks,
-Aleksey