events: return values directly in listeners

Each conditional branch in EventEmitter.prototype.listeners assigns
its return value to a variable ret which is returned at the end.
Instead just return from within each branch. This is both clearer
and more performant.

events/ee-listeners.js n=5000000      3.65 %        *** 3.359171e-10

PR-URL: nodejs/node#16212
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

lib/events.js

@@ -481,23 +481,19 @@ EventEmitter.prototype.removeAllListeners =
     };
 
 EventEmitter.prototype.listeners = function listeners(type) {
-  var evlistener;
-  var ret;
-  var events = this._events;
+  const events = this._events;
 
   if (events === undefined)
-    ret = [];
-  else {
-    evlistener = events[type];
-    if (evlistener === undefined)
-      ret = [];
-    else if (typeof evlistener === 'function')
-      ret = [evlistener.listener || evlistener];
-    else
-      ret = unwrapListeners(evlistener);
-  }
+    return [];
 
-  return ret;
+  const evlistener = events[type];
+  if (evlistener === undefined)
+    return [];
+
+  if (typeof evlistener === 'function')
+    return [evlistener.listener || evlistener];
+
+  return unwrapListeners(evlistener);
 };
 
 EventEmitter.listenerCount = function(emitter, type) {