[lworld] RFR: 8354529: [lworld] Add tests for flat arrays in CDS archive

[Matias Saavedra Silva]

With CDS and AOT now fully supported in lworld, we need better test coverage to ensure that flat arrays are archived correctly. This patch adds a test to check that an array of both a migrate value class and a flattenable user defined value class can be archived and restored correctly in a static CDS dump. 

During the process it was discovered that the object subgraph was overlooking the element class of flat arrays, so proper handling was added courtesy of @iklam.

Verified with tier 1-5 tests.

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Commit messages:
 - Fixed crash and removed from test group
 - Merge branch 'lworld' into flat_array_cds_8354529
 - Ioi fix and enabled second test case
 - 8354529: [lworld] Add tests for flat arrays in CDS archive

Changes: https://git.openjdk.org/valhalla/pull/2081/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2081&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354529
  Stats: 182 lines in 4 files changed: 181 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/2081.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2081/head:pull/2081

PR: https://git.openjdk.org/valhalla/pull/2081