Your code is

    int x;
    if(x == 0) foo();
    if(x != 0) foo();

That is reading the uninitialized value twice. Since it is unspecified, it does not have to be consistent, so you could get the same behavior as non-zero for the first reading, and zero for the second reading. Changing your code to:

    int x;
    if(x == 0) foo();
    else foo();

will give different output (same if you use !=).