[lworld] RFR: 8370217: [lworld] Crash in add_migrated_value_classes after merging JDK-8350550

[Matias Saavedra Silva]

The method add_migrated_value_classes() is being called in non-preview mode leading to an assert due to the expected classes not being present in the AOT cache. The check for `EnableValhalla` is replaced with a check for preview mode. Verified with tier 1-5 tests.

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Commit messages:
 - 8370217: [lworld] Crash in add_migrated_value_classes after merging JDK-8350550

Changes: https://git.openjdk.org/valhalla/pull/1698/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1698&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370217
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/1698.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1698/head:pull/1698

PR: https://git.openjdk.org/valhalla/pull/1698