[lworld] RFR: 8350283: [lworld] Layout helper can have incorrect is_null_free bit for flat arrays

[Frederic Parain]

Small fix to correct the generation of the layout helper for flat arrays using the NULLABLE_ATOMIC_FLAT layout.

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Commit messages:
 - Fix layout helper for flat arrays

Changes: https://git.openjdk.org/valhalla/pull/1370/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1370&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350283
  Stats: 18 lines in 1 file changed: 9 ins; 5 del; 4 mod
  Patch: https://git.openjdk.org/valhalla/pull/1370.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1370/head:pull/1370

PR: https://git.openjdk.org/valhalla/pull/1370