I think there were some older mainframes (CDC? System 360?) that may have done this to some extent -- it'd only really be feasible up to a certain (very low) po

I think there were some older mainframes (CDC? System 360?) that may have done this to some extent -- it'd only really be feasible up to a certain (very low) point though, since you have the potential for an exponential explosion in the number of possible execution paths if more branches come up before you've resolved the outcome of one you're speculating on (especially in the deeper pipelines of modern machines, with perhaps 3- or 4-wide issue and a dozen or so cycles before a branch gets resolved).