[llvm] Ensure `collectTransitivePredecessors` returns Pred only from the Loop. (PR #113831)

[Manish Kausik H via llvm-commits]

Nirhar wrote:

I'm not sure how I can write a lit test for this, reason being, most of the loop transformation passes run `loop-simplify` which removes the irreducibility from the above example. I don't know how to invoke `collectTransitivePredecessors` without using any of the loop passes.

https://github.com/llvm/llvm-project/pull/113831