RFR: 8288129: Shenandoah: Skynet test crashed with iu + aggressive

[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

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

Commit messages:
 - 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: https://git.openjdk.org/jdk/pull/9494/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9494&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288129
  Stats: 8 lines in 1 file changed: 1 ins; 2 del; 5 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