Maxperson
Morkus from Orkus
According to JC when and if statements are interchangeable in 5e rules.
It really wouldn't matter for this, though. If you take the attack action, or when you take the attack action, the action hasn't been taken until it's over. Specific beats general allows you to move in-between your attacks, but Shield Master doesn't have specific language that would allow it to be used before the attack action completes, and you haven't even begun to take the action until the first attack has happened. It's just a declared action until you swing for the first time.
It's also just common sense that you couldn't shove someone hard enough with a shield to knock them over or push them 5 feet away, and also attack with a weapon hard enough to kill someone simultaneously. The body just doesn't work that way.