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

[Matthias Braun via llvm-commits]

MatzeB wrote:

Oh I see, this is for the case where the condition can be constant folded. I guess this makes sense in that case and you want to avoid an underflow? (may be good to mention in a comment/commit message).

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