RFR: 8288129: Shenandoah: Skynet test crashed with iu + aggressive [v3]

[Zhengyu Gu]

> Please review this patch to fix Loom test crash with Shenandoah IU mode.
> 
> Test:
> 
> - [x] hotspot_gc_shenandoah
> - [x] jdk_loom with Shenandoah + IU
> - [x] hotspot_loom with Shenandoah + IU

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Always requires barrier for IU

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9494/files
  - new: https://git.openjdk.org/jdk/pull/9494/files/6f901a74..067b51eb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9494&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9494&range=01-02

  Stats: 3 lines in 1 file changed: 1 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9494.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9494/head:pull/9494

PR: https://git.openjdk.org/jdk/pull/9494