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

[Matt Arsenault via llvm-commits]

arsenm wrote:

>  It feels safer to change the default behavior and allow existing target maintainers to opt-in if and when they chose to. 

LLVM suffers from this mentality. Target maintainers will *not* do this. Nobody will ever notice this option exists. We have a huge list of junk controls that will rarely if ever be discovered or implemented. Everyone is better off with universal infrastructure changes 

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