[jdk19] RFR: 8290250: Shenandoah: disable Loom for iu mode

[Zhengyu Gu]

Please review this trivial patch to disable Loom for Shenandoah iu mode (an experimental feature).

Test:
- [x] hotspot_gc_shenandoah
- [x] jdk_loom with Shenandoah + iu
- [x] hotspot_loom with Shenandoah + iu

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Commit messages:
 - 8290250: Shenandoah: disable Loom for iu mode

Changes: https://git.openjdk.org/jdk19/pull/140/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19&pr=140&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290250
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk19/pull/140.diff
  Fetch: git fetch https://git.openjdk.org/jdk19 pull/140/head:pull/140

PR: https://git.openjdk.org/jdk19/pull/140