RFR: 8259849: Shenandoah: Rename store-val to IU-barrier

[Roman Kennke]

We named the IU barriers StoreValEnqueueBarrier and various similar variations of this. Instead of naming it after how it's implemented, we should name it after its purpose: support IU-marking. It also looks confusing to have enqueue_barrier() and similar stuff, where no barrier is actually enqueued.

The renaming was 100% mechanical.

Testing:
 - [ ] hotspot_gc_shenandoah (x86_64, x86_32, aarch64)

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Commit messages:
 - 8259849: Shenandoah: Rename store-val to IU-barrier

Changes: https://git.openjdk.java.net/jdk/pull/2104/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2104&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259849
  Stats: 108 lines in 23 files changed: 0 ins; 5 del; 103 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2104.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2104/head:pull/2104

PR: https://git.openjdk.java.net/jdk/pull/2104