Use _opt

Author: mshinwell

middle_end/backend_var.ml

@@ -24,7 +24,7 @@ let name_without_opt t =
   let name = name t in
   if String.starts_with ~prefix name
      && String.length name > prefix_len
-  then String.sub name prefix_len (String.length name - prefix_len)
+  then (String.sub name prefix_len (String.length name - prefix_len)) ^ "_opt"
   else name
 
 let name_for_debugger t = name_without_opt t