[lworld] RFR: 8332406: [lworld] C2: Turn flat in array property into proper lattice in type system to address remaining issues [v3]

[Christian Hagedorn]

> This patch turns the boolean flag `_flat_in_array` into a new `FlatInArray` enum to properly define a (Boolean) lattice. This mainly allows us to cleanly represent "maybe flat in array" and "not flat in array". The dedicated top element is the dual of "maybe flat in array".
> 
> To simplify the review and to better understand what the changes are, I added code comments directly in the PR.
> 
> Here is a high-level overview of the changes:
> - `FlatInArray` enum to define a lattice for `flat_in_array`:
> https://github.com/openjdk/valhalla/blob/6b1930c7b9a359223a998d0086a1326a9d7905e7/src/hotspot/share/opto/type.hpp#L1182-L1199
> - Updated `meet` operations accordingly. Needed to be careful when doing the meet above the centerline.
> - Added single `compute_flat_in_array()` method to properly compute the `flat_in_array` property from the information we have when we need to set flat in array newly (this was no properly done before).
> - In `flatten_alias_type()`, we flatten to "maybe flat in array".
> 
> #### Testing:
> - t1-4 + valhalla-comp-stress
> 
> Thanks for your feedback,
> Christian

Christian Hagedorn has updated the pull request incrementally with one additional commit since the last revision:

  Apply suggestions from code review
  
  Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>

-------------

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1774/files
  - new: https://git.openjdk.org/valhalla/pull/1774/files/bf5f2fe4..5e896316

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1774&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1774&range=01-02

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/valhalla/pull/1774.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1774/head:pull/1774

PR: https://git.openjdk.org/valhalla/pull/1774