[llvm] [LoopUnroll] Enable allowexpensivetripcounts when user provides pragma (PR #181267)

[Adel Ejjeh via llvm-commits]

adelejjeh wrote:

@nikic ping — any thoughts on the two options I outlined above? Also, do you have recommendations for other reviewers who would provide good coverage for non-AMD targets?

https://github.com/llvm/llvm-project/pull/181267