[lworld] RFR: 8355397: [lworld] Parse::array_load fails with "array can't be flat" assert

[Quan Anh Mai]

Hi,

The issue here is that `InstanceKlass::flat_array` is too strict and it rejects any class that is atomic and has more than 1 field. I remove the unnecessary restrictions and rename the method to `maybe_flat_in_array` to illustrate the functionality in a clearer manner.

Please kindly review, thanks a lot.

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Commit messages:
 - add tests
 - fix InstanceKlass::flat_array

Changes: https://git.openjdk.org/valhalla/pull/1446/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1446&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355397
  Stats: 57 lines in 16 files changed: 26 ins; 8 del; 23 mod
  Patch: https://git.openjdk.org/valhalla/pull/1446.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1446/head:pull/1446

PR: https://git.openjdk.org/valhalla/pull/1446