In general branching code is faster than branchless code and there's many many places that will demonstrate this with a quick Google. You know how many cycles a correctly predicted branch takes? 0.

On the other hand branchless code has to wait for each calculation to reach a certain stage in the pipeline since the thing to be output is dependent on the result. The CPU will have a whole lot of halts.

So why is this faster? Because the input is literally random(). The branch predictor will be wrong. This isn't normal though. The compiler is creating code that will be faster in most normal use cases.

It's an artificial benchmark that works against the output the compiler produces.