Fix uses of polymorphic comparison in Flambda 2 (oxcaml#2312)

* Add -open Int_replace_polymorphic_compare for flambda2

* Remove explicit opens of Int_replace_polymorphic_compare

* Fix occurrences of polymorphic comparison

Author: mshinwell

middle_end/flambda2/algorithms/dune

@@ -7,5 +7,5 @@
  (flags
   (:standard -principal))
  (ocamlopt_flags
-  (:standard -O3))
+  (:standard -O3 -open Int_replace_polymorphic_compare))
  (libraries ocamlcommon))

middle_end/flambda2/algorithms/lmap.ml

@@ -78,7 +78,7 @@ module Make (T : Thing) : S with type key = T.t = struct
 
   let empty = []
 
-  let is_empty m = m = []
+  let is_empty m = match m with [] -> true | _ :: _ -> false
 
   let add k v m = (k, v) :: m
 
@@ -147,9 +147,9 @@ module Make (T : Thing) : S with type key = T.t = struct
   let of_seq m = List.of_seq m
 
   let print_assoc print_key print_datum ppf l =
-    if l = []
-    then Format.fprintf ppf "{}"
-    else
+    match l with
+    | [] -> Format.fprintf ppf "{}"
+    | _ :: _ ->
       Format.fprintf ppf "@[<hov 1>{%a}@]"
         (Format.pp_print_list ~pp_sep:Format.pp_print_space
            (fun ppf (key, datum) ->

middle_end/flambda2/algorithms/patricia_tree.ml

@@ -12,8 +12,6 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open! Int_replace_polymorphic_compare
-
 (* The following is a "little endian" implementation. *)
 
 (* CR-someday mshinwell: Can we fix the traversal order by swapping endianness?

middle_end/flambda2/bound_identifiers/dune

@@ -18,7 +18,7 @@
    -open
    Flambda2_ui))
  (ocamlopt_flags
-  (:standard -O3))
+  (:standard -O3 -open Int_replace_polymorphic_compare))
  (libraries
   ocamlcommon
   flambda2_algorithms

middle_end/flambda2/classic_mode_types/dune

@@ -17,7 +17,7 @@
    -open
    Flambda2_term_basics))
  (ocamlopt_flags
-  (:standard -O3))
+  (:standard -O3 -open Int_replace_polymorphic_compare))
  (libraries
   ocamlcommon
   flambda2_identifiers

middle_end/flambda2/cmx/dune

@@ -32,7 +32,7 @@
    -open
    Flambda2_ui))
  (ocamlopt_flags
-  (:standard -O3))
+  (:standard -O3 -open Int_replace_polymorphic_compare))
  (libraries
   ocamlcommon
   flambda_backend_utils

middle_end/flambda2/compare/compare.ml

@@ -1,6 +1,5 @@
 [@@@ocaml.warning "-fragile-match"]
 
-open! Int_replace_polymorphic_compare
 open! Flambda
 
 (* General notes on comparison

middle_end/flambda2/compare/dune

@@ -24,5 +24,5 @@
    -open
    Flambda2_terms))
  (ocamlopt_flags
-  (:standard -O3))
+  (:standard -O3 -open Int_replace_polymorphic_compare))
  (libraries ocamlcommon flambda2_algorithms flambda2_terms))

middle_end/flambda2/dune

@@ -32,7 +32,7 @@
    -open
    Flambda2_ui))
  (ocamlopt_flags
-  (:standard -O3))
+  (:standard -O3 -open Int_replace_polymorphic_compare))
  (modules flambda2)
  (libraries
   ocamlcommon

middle_end/flambda2/from_lambda/closure_conversion.ml

@@ -16,7 +16,6 @@
 
 [@@@ocaml.warning "-fragile-match"]
 
-open! Int_replace_polymorphic_compare
 open! Flambda
 module BP = Bound_parameter
 module IR = Closure_conversion_aux.IR