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

[Matthias Braun via llvm-commits]

MatzeB wrote:

I suspect some amount of noise in the sample / mapping samples to actual blocks etc. is unavoidable and I am not sure this situation (that only becomes "bad" when you consider the fact that the particular loop condition cannot be false in the first iteration) will be considered by smoothing algos. So seems fine to me, but leaving final call to @WenleiHe about whether some inaccuracies like this are considered normal (and hence must be worked around).

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