RFR: 8288129: Shenandoah: Skynet test crashed with iu + aggressive [v2]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'master' into JDK-8288129-iu-vthr
 - Fix
 - 8288129: Shenandoah: Skynet test crashed with iu + aggressive
 - Merge branch 'master' into JDK-8288129-iu-vthr
 - Merge branch 'master' into JDK-8288129-iu-vthr
 - v0

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9494/files
  - new: https://git.openjdk.org/jdk/pull/9494/files/24cad3b5..6f901a74

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9494&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9494&range=00-01

  Stats: 996 lines in 46 files changed: 686 ins; 75 del; 235 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