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

[Matt Arsenault via llvm-commits]

arsenm 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.

You can directly run the must-execute analysis printer and check the output. Tests ideally run in isolation, the context where it is used in the pass pipeline should not matter 

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