RFR: 8259849: Shenandoah: Rename store-val to IU-barrier [v3]

[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:

  Change to I-U barrier name in tests

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2104/files
  - new: https://git.openjdk.java.net/jdk/pull/2104/files/3804e64e..31810029

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2104&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2104&range=01-02

  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 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