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

[Roman Kennke]

Looks good to me! Thank you!

> 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
>