Reusing declarations in the reverse pass

[PetroZarytskyi]

This issue is going to arise after landing #655.
Reusing the same declarations for the reverse pass as the ones used in the forward pass does not cause any errors since scopes are currently reused as well. For example, if we differentiate this statement

if (1) {
    double n;
    n *= x;
}

We will get this statement for the forward pass

if (1) {
    double n;
    _t0 = n;
    n *= x;
}

And this one for the reverse pass

if (1) {
    {
        n = _t0;
        double _r_d0 = _d_n;
        _d_n += _r_d0 * x;
        double _r0 = n * _r_d0;
        * _d_x += _r0;
        _d_n -= _r_d0;
        _d_n;
    }
}

Both statements have the same scope and, therefore, using n in the reverse pass does not cause an error. However, compiling the dumped code by hand obviously doesn't work.
Ideally, this should be solved by placing a separate declaration statement in the reverse pass. This would cause a couple of complications:

1. The declaration should be inserted before the variable is used and that might be tricky because the order of statements in the reverse pass is reversed.

2. We should store the last value of a variable in the forward pass before it goes out of scope and then assign the corresponding variable in the reverse pass with that value. Also, it would be better to do this only when the value is useful (use TBR analysis).

   There is also an easier temporary solution to move the declaration to function globals and place an assignment instead of an initialization. For example this code

if (int k = 1) {
...

Can be replaced with

int k;
if (k = 1) {
...

Obviously, this will harm performance and decrease readability.