[llvm] LoopIdiomRecognize: detect and convert powi idiom (PR #72650)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> * I'm somewhat skeptical that this is going to be a useful optimization in practice. I don't see the kind of loops where this transformation would be beneficial just coming up all that frequently, even as something that might be the result of optimization.

Addressed all other review comments. I agree that this is quite a minor optimization, and that this idiom doesn't crop up frequently; nevertheless, it is a valid optimization, and benefits some programs.

https://github.com/llvm/llvm-project/pull/72650