AbstractInterpreter: less garbage

Summary:
Replace `map |> filter_opt |> fold` with one `fold`.
Also preparing for future changes...

Reviewed By: ngorogiannis

Differential Revision: D8185643

fbshipit-source-id: 4d311eb

infer/src/absint/AbstractInterpreter.ml

@@ -104,12 +104,17 @@ struct
   let rec exec_worklist cfg work_queue inv_map proc_data ~debug =
     let compute_pre node inv_map =
       let extract_post_ pred = extract_post (CFG.id pred) inv_map in
-      let all_posts = List.map ~f:extract_post_ (CFG.preds cfg node) in
-      match List.filter_opt all_posts with
-      | post :: posts ->
-          Some (List.fold ~f:Domain.join ~init:post posts)
-      | [] ->
-          None
+      CFG.preds cfg node
+      |> List.fold ~init:None ~f:(fun joined_post_opt pred ->
+             match extract_post_ pred with
+             | None ->
+                 joined_post_opt
+             | Some post as some_post ->
+               match joined_post_opt with
+               | None ->
+                   some_post
+               | Some joined_post ->
+                   Some (Domain.join joined_post post) )
     in
     match Scheduler.pop work_queue with
     | Some (_, [], work_queue') ->