Consider a redesign of the for loop's body in reverse pass

[kchristin22]

The current implementation of the loop's body in the reverse pass of the Reverse mode is not very clear. Consider this example:

double fn(double u, double v) {
    double res = 11 * u;
    for (int i = 0; i < 3; i++) {
        if (i==2){
            res += u * i;
            continue;
        }
        else {
            res -= u * i;
        }
        res += u;
    }
    return res;
}

The for loop in the reverse pass, after the fix of #710, is derived like so:

for (; _t0; _t0--) {
        i--;
        switch (clad::pop(_t4)) {
          case 2UL:
            ;
            {
                res = clad::pop(_t6);
                double _r_d2 = _d_res;
                * _d_u += _r_d2;
            }
            if (clad::pop(_t2)) {
              case 1UL:
                ;
                {
                    res = clad::pop(_t3);
                    double _r_d0 = _d_res;
                    * _d_u += _r_d0 * i;
                    _d_i += u * _r_d0;
                }
            } else {
                {
                    res = clad::pop(_t5);
                    double _r_d1 = _d_res;
                    * _d_u += -_r_d1 * i;
                    _d_i += u * -_r_d1;
                }
            }
        }

Even though the results are correct and the cases handled as expected, the indexing of the code is not very traditional.

The proposed change evaluates this snippet as:

for (; _t0; _t0--) {
        i--;
        switch (clad::pop(_t4)) {
          case 2UL:
            {
                res = clad::pop(_t6);
                double _r_d2 = _d_res;
                * _d_u += _r_d2;
                if (clad::pop(_t2))
                    ;
                else {
                    {
                        res = clad::pop(_t5);
                        double _r_d1 = _d_res;
                        * _d_u += -_r_d1 * i;
                        _d_i += u * -_r_d1;
                    }
                }
                break;
            }
          case 1UL:
            {
                res = clad::pop(_t3);
                double _r_d0 = _d_res;
                * _d_u += _r_d0 * i;
                _d_i += u * _r_d0;
                break;
            }
        }