Fix T75574: Instant crash: averaging custom normals on model.

Fairly critical code mistake actually, since it uses malloca,
BLI_smallstack should *never* be declared inside a loop...

Also optimized handling of the `loop_weight` heapsimple, we can also
only create and use a single one for all edited objects...

Found two other operators potentially affected by same issue (split
normals, and weld edges into faces).

source/blender/editors/mesh/editmesh_intersect.c

@@ -704,6 +704,8 @@ static int edbm_face_split_by_edges_exec(bContext *C, wmOperator *UNUSED(op))
   BMEdge *e;
   BMIter iter;
 
+  BLI_SMALLSTACK_DECLARE(loop_stack, BMLoop *);
+
   ViewLayer *view_layer = CTX_data_view_layer(C);
   uint objects_len = 0;
   Object **objects = BKE_view_layer_array_from_objects_in_edit_mode_unique_data(
@@ -717,8 +719,6 @@ static int edbm_face_split_by_edges_exec(bContext *C, wmOperator *UNUSED(op))
       continue;
     }
 
-    BLI_SMALLSTACK_DECLARE(loop_stack, BMLoop *);
-
     {
       BMVert *v;
       BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {