Support statements in ITE

[katat]

In order to support the following calculation:

// 1 if true, 0 if false
function long_gt(n, k, a, b) {
    for (var i = k - 1; i >= 0; i--) {
        if (a[i] > b[i]) {
            return 1;
        }
        if (a[i] < b[i]) {
            return 0;
        }
    }
    return 0;
}

The compiler should support statements and early returns in the ITE block, otherwise it is too tricky to implement the logic like this. Perhaps at least we should enhance this feature in the hint functions.

[mimoo]

agree that in the hint functions we should allow anything (early return, early break, large if-then-else statements, etc.)

in the rest IMO we should not for now, as it would be hard to support other backends than r1cs