nir/dominance: don't allocate 0-sized dom_children

86% of all ralloc calls for dom_children in Unigine Heaven + Superposition
had size == 0. It was only allocating the ralloc header.

It was 6.1% of all ralloc calls with the gallium noop driver.

Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36728>

src/compiler/nir/nir_dominance.c

@@ -135,6 +135,10 @@ calc_dom_children(nir_function_impl *impl)
    }
 
    nir_foreach_block_unstructured(block, impl) {
+      if (!block->num_dom_children) {
+         block->dom_children = NULL;
+         continue;
+      }
       block->dom_children = ralloc_array(mem_ctx, nir_block *,
                                          block->num_dom_children);
       block->num_dom_children = 0;