[llvm] [LoopRotate] Set loop back edge weight to not less than exit weight (PR #86496)

[Matthias Braun via llvm-commits]

MatzeB wrote:

I don't follow why this would be a situation that needs fixup. While it is less common, it is perfectly fine for a loop to exit more often than using the loop backedge (when 0-trip counts happen often enough for a given loop to push the average trip count below 1).

https://github.com/llvm/llvm-project/pull/86496