RFR: 8259849: Shenandoah: Rename store-val to IU-barrier [v2]

[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)

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Update copyright years in modified files

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2104/files
  - new: https://git.openjdk.java.net/jdk/pull/2104/files/2d2b0130..3804e64e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2104&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2104&range=00-01

  Stats: 20 lines in 20 files changed: 0 ins; 0 del; 20 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