flambda-backend: Revert "Revert bswap PRs (480 and 482)" (#506)

* Revert "Revert bswap PRs (480 and 482) (#501)"

This reverts commit 52cd50e.

* Test for bswap32

* Add more test inputs

* Refactor test

* Add more tests

Author: gretay-js

testsuite/tests/prim-bswap/bswap32roundtrip.ml

@@ -0,0 +1,34 @@
+(* TEST
+*)
+type buf = bytes
+
+external create_buf : int -> buf = "caml_create_bytes"
+external unsafe_get32 : buf -> int -> int32 = "%caml_bytes_get32u"
+external unsafe_set32 : buf -> int -> int32 -> unit = "%caml_bytes_set32u"
+external bswap32 : int32 -> int32 = "%bswap_int32"
+
+let buf = create_buf 32
+let read buf ~pos = unsafe_get32 buf pos |> bswap32 |> Stdlib.Int32.to_int
+let write buf ~pos n = unsafe_set32 buf pos (n |> Stdlib.Int32.of_int |> bswap32)
+
+let numbers =
+    [ 0x11223344l
+    ; 0xf0f0f0f0l
+    ; 0l
+    ; -1l
+    ; 0xf0fedcbal
+    ; 0x0fedfcbal
+    ; 0x7fedfcbal
+    ; Stdlib.Int32.max_int
+    ; Stdlib.Int32.min_int
+    ]
+  ;;
+
+let test n =
+  let n = Stdlib.Int32.to_int n in
+  write buf ~pos:0 n;
+  let n' = read buf ~pos:0 in
+  assert (Int.equal n n')
+
+let () =
+  List.iter test numbers