Fix global flow analysis (see #1556)

It needs to take into account values flowing out of the toplevel function.

Author: vouillon

compiler/lib/global_deadcode.ml

@@ -253,15 +253,6 @@ let liveness prog pure_funs (global_info : Global_flow.info) =
     | Stop | Branch _ | Poptrap _ | Pushtrap _ -> ()
   in
   Addr.Map.iter (fun _ block -> live_block block) prog.blocks;
-  Code.traverse
-    { Code.fold = Code.fold_children }
-    (fun pc () ->
-      match Addr.Map.find pc prog.blocks with
-      | { branch = Return x, _; _ } -> add_top x
-      | _ -> ())
-    prog.start
-    prog.blocks
-    ();
   live_vars
 
 (* Returns the set of variables given a table of variables. *)

compiler/lib/global_flow.ml

@@ -233,7 +233,16 @@ let expr_deps blocks st x e =
       cont_deps blocks st cont
   | Field (y, _, _) -> add_dep st x y
 
-let program_deps st { blocks; _ } =
+let program_deps st { start; blocks; _ } =
+  Code.traverse
+    { Code.fold = Code.fold_children }
+    (fun pc () ->
+      match Addr.Map.find pc blocks with
+      | { branch = Return x, _; _ } -> do_escape st Escape x
+      | _ -> ())
+    start
+    blocks
+    ();
   Addr.Map.iter
     (fun _ block ->
       List.iter block.body ~f:(fun (i, _) ->