Tweak test to make allocation toplevel in classic mode

(cherry picked from commit ad845a1)

Author: lthls

ocaml/testsuite/tests/mixed-blocks/structural_constants.ml

@@ -58,6 +58,14 @@ let () = print_endline "Success!"
 let () = print_endline "Testing optimization (this is a no-op in bytecode)..."
 
 let () =
+  (* Both classic mode and optimized mode should be able to statically
+     allocate the inconstant list. *)
+  let s = Sys.opaque_identity "foo" in
+  let bytes_start0 = Gc.allocated_bytes () in
+  let bytes_start1 = Gc.allocated_bytes () in
+  let _ =
+    Sys.opaque_identity [A #4.0; B ("B", #5.0); C ("C", #6.0, 6); D s]
+  in
   match Sys.backend_type with
   | Bytecode -> ()
   | Native | Other _ ->
@@ -74,14 +82,6 @@ let () =
       == (Sys.opaque_identity create_a) ());
     assert ((Sys.opaque_identity create_vs) ()
       == (Sys.opaque_identity create_vs) ());
-    (* Both classic mode and optimized mode should be able to statically
-       allocate the inconstant list. *)
-    let s = Sys.opaque_identity "foo" in
-    let bytes_start0 = Gc.allocated_bytes () in
-    let bytes_start1 = Gc.allocated_bytes () in
-    let _ =
-      Sys.opaque_identity [A #4.0; B ("B", #5.0); C ("C", #6.0, 6); D s]
-    in
     let bytes_end = Gc.allocated_bytes () in
     assert (bytes_start0 +. bytes_end = 2. *. bytes_start1)
 ;;