^Yes and no. On the one hand it is a personification of the Geth and thus not a true individual being that can be killed (short of a galaxy-wide cataclysm), on the other, if that platform is destroyed during the suicide mission, it says something to the effect that an attempted upload has failed. I think it's fair to assume means that all of it's stored memories since it's last synch with the Geth are lost. Either way, depending on what Shepard did or didn't do with the heretics, I think all bets are off as to how the Geth may change as a whole in ME3 and even if Legion survives it should also change to reflect that.
Whatever happens, I think it's unlikely that if Legion was lost that the Geth will simply send a new custom mobile platform to join the crew.