[llvm] [LoopPassManager] Exit early for loop-free fns in FunctionToLoopPass. (PR #215011)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

Looks like this still needs some more test cleanup, but this seems like a really nice direction to me.

LGTM once CI passes.

https://github.com/llvm/llvm-project/pull/215011