This depends entirely on the specific code you're talking about. If the compiler finds out a specific optimization is applicable for a certain block of code, then it uses it.
So your question cannot be answered. Only possible answer is "it depends". Another guess: "From nothing to an order of magnitude. Probably most cases about 5-20%."
Yeah, I realize it was an open-ended question. I'm just curious if one was starting a (C-like) compiler from scratch, where the biggest bang for the buck comes from. It seems like peephole optimizations are already handled at runtime by the CPU itself.
So your question cannot be answered. Only possible answer is "it depends". Another guess: "From nothing to an order of magnitude. Probably most cases about 5-20%."