readline: fix permanent deoptimizations

PR-URL: https://github.com/nodejs/node/pull/12456
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/readline.js

@@ -449,8 +449,8 @@ Interface.prototype._tabComplete = function(lastKeypressWasTab) {
       }
 
       // If there is a common prefix to all matches, then apply that portion.
-      const f = completions.filter(function(e) { if (e) return e; });
-      const prefix = commonPrefix(f);
+      var f = completions.filter(function(e) { if (e) return e; });
+      var prefix = commonPrefix(f);
       if (prefix.length > completeOn.length) {
         self._insertString(prefix.slice(completeOn.length));
       }