Would there even be a Syrian Civil War if the Soviets were still around? The balance of power in the world would be so completely different from our timeline that you have to answer that question before you can get to yours.
My own feeling is that assuming the Soviets are still around, Syria will most likely be in their sphere of influence and many of the shenanigans involving arming rebels would be a non-starter. The whole Arab Spring might not happen or be in a far more limited version than what we had. Even taking aside the specter of nuclear war in an open conflict, I think the Soviet Navy would be an effective deterrent from large scale supplying of rebel forces. The Turks are going to be a lot more wary of mucking around in the whole business. Assad's military would be far better supplied in turn, increasing its effectiveness.
So in other words, no the SU would not intervene because as a superpower they would have many other options besides brute intervention. It would take a leader of tremendous ineptitude to have let it get that far.