nir/lower_variables: Use a real dominance DFS for variable renaming

nir/lower_variables: Use a real dominance DFS for variable renaming

Previously, we were just iterating over the program "in order" which
kind-of approximates a DFS, but not really.  In particular, we got the
following case wrong:

loop {
   a = 3;
   if (foo) {
      a = 5;
   } else {
      break;
   }
   use(a);
}

where use(a) would get 3 instead of 5 because of premature popping of the
SSA def stack.  Now, since we do an actaul DFS, we should evaluate use(a)
immediately after a = 5 and we should be ok.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>