nir: unrolling improvements

This series was written with the aim to unroll a loop in a glamor shader, I think its a nice additional feature set to loop unrolling.

shaders/glamor/82.shader_test

    uniform float stops[18];
    int i = 0;
 
      ... 
 
    for(i = 0; i < n_stop - 1; i++) {
        if(stop_len < stops[i])
            break; 
    }

      ... more use of i