[llvm] [LoopUnroll] Use branch probability in multi-exit loop unrolling (PR #164799)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

High level comment - why is fixing BPIs stability here not an option?
_(note: will be slow with reviews for the remaining of the week - took a long weekend)_

https://github.com/llvm/llvm-project/pull/164799