if(n!=0) n=0; v/s n=0; Which is more efficient and why?

While typing a program as a high level programmer, n=0; looks more efficient and clean. But is it really that n=0; is more efficient than if(n!=0) n=0;?

Case 1: When n is more likely to be 0.

Case 2: When n is less likely to be 0.

Case 3: When n is absolutely uncertain.