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

[Ashutosh Mehra]

Please review this patch to fix the crash when running Loom with Shenandoah in iu+aggressive mode.

1. Always enable barrier for Shenandoah IU mode (this change is same as https://github.com/openjdk/jdk/pull/9494), but is not sufficient to prevent the assertion failure.
2. Run barriers for oops in stack chunk header. The code in `stackChunkOopDesc::do_barriers` is missing barriers on the oops present in the stack chunk header.

Signed-off-by: Ashutosh Mehra <asmehra at redhat.com>

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Commit messages:
 - 8288129: Shenandoah: Skynet test crashed with iu + aggressive

Changes: https://git.openjdk.org/jdk/pull/9982/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9982&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288129
  Stats: 39 lines in 3 files changed: 39 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9982.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9982/head:pull/9982

PR: https://git.openjdk.org/jdk/pull/9982