(He also probably correctly thinks it is more likely that Harry can rescue him from being frozen than he could rescue Harry.)

As long as Quirrellmort doesn't kill Harry as soon as he gets the stone, which seems like a rather salient possibility. Assuming Dumbledore was acting rationally, we can take this as evidence that Dumbledore knew there was a reason why Quirrellmort would want to keep Harry alive, though it's also likely that Dumbledore was just being impulsively selfless in sacrificing himself.