nir/dominance: Handle unreachable blocks

nir/dominance: Handle unreachable blocks

Previously, nir_dominance.c didn't properly handle unreachable blocks.
This can happen if, for instance, you have something like this:

loop {
   if (...) {
      break;
   } else {
      break;
   }
}

In this case, the block right after the if statement will be unreachable.
This commit makes two changes to handle this.  First, it removes an assert
and allows block->imm_dom to be null if the block is unreachable.  Second,
it properly skips unreachable blocks in calc_dom_frontier_cb.