[lworld] RFR: 8370880: [lworld] Split ciObjArrayKlass into ciRefArrayKlass and ciFlatArrayKlass

[Quan Anh Mai]

Hi,

This PR splits `ciObjArrayKlass` into `ciRefArrayKlass` and `ciFlatArrayKlass`, aligns the hierarchy with the corresponding types of the VM.

I also had to fix `GraphKit::cast_to_flat_array`, currently it incorrectly assumes that the cast is to an exact type, splitting into `cast_to_flat_array` and `cast_to_flat_array_exact` seems to make it clearer.

Please kindly review, thanks a lot.

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Commit messages:
 - Refactor ciObjArrayKlass to align with the VM type

Changes: https://git.openjdk.org/valhalla/pull/1739/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1739&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370880
  Stats: 407 lines in 21 files changed: 165 ins; 170 del; 72 mod
  Patch: https://git.openjdk.org/valhalla/pull/1739.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1739/head:pull/1739

PR: https://git.openjdk.org/valhalla/pull/1739