[llvm] [SCEV] Collect and merge loop guards through PHI nodes with multiple incoming values (PR #113915)

[Florian Hahn via llvm-commits]

fhahn wrote:

It would be good to adjust the description to mention that this helps to determine tighter bounds on the trip counts of scalar tail loops after vectorization, helping to avoid unnecessary transforms on them (would also be good to link the motivating epilogue vectorization change)

https://github.com/llvm/llvm-project/pull/113915