Oh, I see what you mean -- I was talking (and thinking) about the unrolled version so it didn't make sense how speculation could help there. But I imagine that typically the kind of long chains that you might want to do in parallel in a single basic block are perhaps something that wouldn't get executed that far after a branch, if the only purpose is to not waste time after a branch misprediction. Plus from what I understand you'd still be wasting execution units here, just not by idling them but rather by speculating the "I'm done" branch repeatedly.

EDIT: I just found that the idea that I had in my head actually exists and is called "modulo scheduling".