flambda-backend: Fix valid character range computation for compilatio…

…n units (#1110)

utils/compilation_unit.ml

@@ -138,14 +138,10 @@ end = struct
   end)
 
   let is_valid_character first_char c =
-    let code = Char.code c in
-    if first_char
-    then code >= 65 && code <= 90 (* [A-Z] *)
-    else
-      Char.equal c '_'
-      || (code >= 48 && 57 <= 90 (* [0-9] *))
-      || (code >= 65 && code <= 90 (* [A-Z] *))
-      || (code >= 97 && code <= 122 (* [a-z] *))
+    match c with
+    | 'A' .. 'Z' -> true
+    | '_' | '0' .. '9' | 'a' .. 'z' -> not first_char
+    | _ -> false
 
   let parse_for_pack pack =
     let prefix = String.split_on_char '.' pack in