nir/opt_if: don't mark progress if nothing changes

if we have something like this:

loop {
   ...
   if x {
      break;
   } else {
      continue;
   }
}

opt_if_loop_last_continue returns true marking progress allthough nothing
changes.

Fixes: 5921a19d4b "nir: add if opt opt_if_loop_last_continue()"
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir_opt_if.c

@@ -844,6 +844,13 @@ opt_if_loop_last_continue(nir_loop *loop)
    if (!then_ends_in_continue && !else_ends_in_continue)
       return false;
 
+   /* if the block after the if/else is empty we bail, otherwise we might end
+    * up looping forever
+    */
+   if (&nif->cf_node == nir_cf_node_prev(&last_block->cf_node) &&
+       exec_list_is_empty(&last_block->instr_list))
+      return false;
+
    /* Move the last block of the loop inside the last if-statement */
    nir_cf_list tmp;
    nir_cf_extract(&tmp, nir_after_cf_node(if_node),