[nikic]

JITs are generally one of the most challenging places to use LLVM, exactly because of its bad compile-time characteristics. There are some successful uses of LLVM based JIT compilers (e.g. Azul's Falcon JIT), but this is definitely a use case where you can't just use the standard optimization pipeline. You'll generally use a custom pipeline and likely only use LLVM for the second stage JIT compiler.

That said, I don't think your statement that LLVM optimizations only benefit you if you generate bad input IR is correct. It just sounds like they are not useful for your specific problem domain.