`a = num; a += 1; if (num < a)` is the same as `if (num < (num + 1))`, which for unsigned integer addition can be rewritten as `if (num != UINT_MAX)`. So there's no need to actually compute `a+1`, the comparison is against a constant.

If the code returns `num * a` then the value of `a` is now necessary, and must be computed before the function returns.

For signed integer addition the compiler is allowed to assume that `(num < (num + 1))` is true, so the comparison can be removed entirely.