avoid removing spec of `x = x` assignments

src/optimize_machine.ml

@@ -784,7 +784,15 @@ let rec instr_is_skip instr =
 
 and instrs_are_skip instrs = List.for_all instr_is_skip instrs
 
-let instr_cons instr cont = if instr_is_skip instr then cont else instr :: cont
+let instr_cons instr cont =
+  match instr_is_skip instr, cont with
+  | true, [] -> [ { instr with instr_desc = MComment "" } ]
+  | true, i :: cont -> { i with instr_spec = i.instr_spec @ instr.instr_spec } :: cont
+  | false, _ -> instr :: cont
+  (* let instr_desc = *)
+  (*   if instr_is_skip instr then MComment "" else instr.instr_desc in *)
+  (* let instr = { instr with instr_desc } in *)
+  (* instr :: cont *)
 
 (* XXX: UNUSED *)
 (* let rec instr_remove_skip instr cont =
@@ -1465,7 +1473,8 @@ let optimize params prog node_schs machine_code =
       let reuse_tables = Scheduling.compute_prog_reuse_table node_schs in
       machine_code
       |> machines_reuse_variables reuse_tables
-      |> machines_fusion |> machines_clean
+      |> machines_fusion
+      |> machines_clean
     else machine_code
   in