[llvm] [LV] Avoid SCEVChecks when IV update doesn't overflow (PR #171605)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

Thanks for the guidance: I'm finding it hard to reason about these checks, which was the reason for the incorrect generalization. I'm still confused about why we need IV update overflow checks when VF is not scalable: I factored out some existing logic and re-used it in the redo of the patch, and I'm not able to reason about it fully.

https://github.com/llvm/llvm-project/pull/171605