[jdk16u] RFR: 8259849: Shenandoah: Rename store-val to IU-barrier

[Aleksey Shipilev]

This improves Shenandoah UX and prepares ground for future backports.

Additional testing:
 - [x] hotspot_gc_shenandoah

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Commit messages:
 - Backport e60c9926993d530fabf92d22b1e92b89bd850c51

Changes: https://git.openjdk.java.net/jdk16u/pull/59/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16u&pr=59&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259849
  Stats: 131 lines in 26 files changed: 0 ins; 5 del; 126 mod
  Patch: https://git.openjdk.java.net/jdk16u/pull/59.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16u pull/59/head:pull/59

PR: https://git.openjdk.java.net/jdk16u/pull/59