[lworld] RFR: 8378780: [lworld] Assert in Node::depends_only_on_test_impl due to CastI2N

[Tobias Hartmann]

After [JDK-8347365](https://bugs.openjdk.org/browse/JDK-8347365) got merged in, the Valhalla specific `CastI2NNode` needs to define `depends_only_on_test_impl`.

Thanks,
Tobias

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Commit messages:
 - JDK-8378780

Changes: https://git.openjdk.org/valhalla/pull/2179/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2179&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378780
  Stats: 7 lines in 2 files changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/2179.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2179/head:pull/2179

PR: https://git.openjdk.org/valhalla/pull/2179