nir/opt_if: also merge break statements with ones after the branch

nir/opt_if: also merge break statements with ones after the branch

 This optimizations turns

     loop {
        ...
        if (cond1) {
           if (cond2) {
              do_work_1();
              break;
           } else {
              do_work_2();
           }
           do_work_3();
           break;
        } else {
           ...
        }
     }

 into:

     loop {
        ...
        if (cond1) {
           if (cond2) {
              do_work_1();
           } else {
              do_work_2();
              do_work_3();
           }
           break;
        } else {
           ...
        }
     }

As this optimizations moves code into the NIF statement,
it re-iterates on the branch legs in case of success.

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7587>