[llvm] [DA] Move constraint propagation under flag disabled by default (PR #160924)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj commented:

I think this feature should be removed, not just disabled behind a flag, for the following reasons:

- I don't think it will ever be re-enabled due to its correctness issues. Fixing those would require non-trivial changes.
- This feature doesn't seem particularly useful, as you can see almost no regression tests are affected, and notably Propagating.ll remains unchanged.
- It adds a fair amount of code complexity, making it harder to reason about other parts of DA. For instance, without it, we might no longer need to distinguish between `Separable` and `Coupled`.
- Fixing the correctness issues would require limiting its current capabilities, which would make it even less useful.

Given these points, I don't think it's worth keeping. The maintenance cost would likely outweigh any potential benefit.

https://github.com/llvm/llvm-project/pull/160924