LLVM is literally designed as an production quality optimisation/code-generation backend, so it's seems perfectly reasonable for projects to rely on LLVM's opti

LLVM is literally designed as an production quality optimisation/code-generation backend, so it's seems perfectly reasonable for projects to rely on LLVM's optimisations without doing their own, especially when there are other things people can work on than the large effort to optimise some programs slightly better (and even fewer programs a lot better, but these are often microbenchmarks, or tight loops like the OP that can be diagnosed with a profiler relatively easily and rewritten) by doing language-specific optimisations. It's definitely a good long-term goal, but it requires a lot of infrastructure, and ends up requiring duplicating a lot of the optimisations and analysis already in LLVM (e.g. inlining is a critical transformation for enabling other optimisations) before one can get great results from the custom optimisations. In any case, compiler optimisations are usually easy to implement on specific styles of internal representations, and this is MIR for Rust, which was only introduced recently, after requiring a very large refactoring of the compiler's internal pipeline.