[lworld] RFR: 8319967: [lworld] TypeAryPtr::cast_to_not_null_free hits assert(!not_null_free || !is_flat()) failed: inconsistency

[Tobias Hartmann]

I hit this very intermittent failure with stress testing and was able to extract a simple test. [JDK-8227634](https://bugs.openjdk.org/browse/JDK-8227634) added the following optimization: When casting an element loaded from an array to a known type, we use that type information to set not-null-free not-flat properties of the array (also when storing an object of a known type to an array). Now that optimization gets confused when we load a field of an element from a flat array. In the failing case (see `TestArrays::test150`), we load an Object field from a flat array element and cast it to String. The optimization then incorrectly assumes that we are casting the (primitive type) element and casts the array to not-flat which fails because the array is known to be flat. The actual fix is the additional `!ary_t->is_flat()` check in `GraphKit::gen_checkcast`. The rest is refactoring that changes `flat_array` to `flat_in_array` which is more precise.

Thanks,
Tobias

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Commit messages:
 - 8319967: [lworld] TypeAryPtr::cast_to_not_null_free hits assert(\!not_null_free || \!is_flat()) failed: inconsistency

Changes: https://git.openjdk.org/valhalla/pull/951/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=951&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319967
  Stats: 111 lines in 13 files changed: 11 ins; 1 del; 99 mod
  Patch: https://git.openjdk.org/valhalla/pull/951.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/951/head:pull/951

PR: https://git.openjdk.org/valhalla/pull/951