Fix performance issue with Structure.is_merge_node

Author: vouillon

compiler/lib/structure.ml

@@ -42,10 +42,10 @@ let is_forward g pc pc' = Hashtbl.find g.block_order pc < Hashtbl.find g.block_o
 let is_merge_node' block_order preds pc =
   let s = try Hashtbl.find preds pc with Not_found -> Addr.Set.empty in
   let o = Hashtbl.find block_order pc in
-  let n =
-    Addr.Set.fold (fun pc' n -> if Hashtbl.find block_order pc' < o then n + 1 else n) s 0
-  in
-  n > 1
+  match Addr.Set.find_first_opt (fun pc' -> Hashtbl.find block_order pc' < o) s with
+  | None -> false
+  | Some pc' ->
+      Addr.Set.exists (fun pc'' -> pc'' <> pc' && Hashtbl.find block_order pc'' < o) s
 
 let empty_body body =
   List.for_all