Simplify reduce wrapper to just determine initial

Well, and optimize `iteratee`, cause we don't really need context
anywhere else.

underscore.js

@@ -183,7 +183,14 @@
   var createReduce = function(dir) {
     // Optimized iterator function as using arguments.length
     // in the main function will deoptimize the, see #1991.
-    var iterator = function(obj, iteratee, memo, keys, index, length) {
+    var reducer = function(obj, iteratee, memo, initial) {
+      var keys = !isArrayLike(obj) && _.keys(obj),
+          length = (keys || obj).length,
+          index = dir > 0 ? 0 : length - 1;
+      if (!initial) {
+        memo = obj[keys ? keys[index] : index];
+        index += dir;
+      }
       for (; index >= 0 && index < length; index += dir) {
         var currentKey = keys ? keys[index] : index;
         memo = iteratee(memo, obj[currentKey], currentKey, obj);
@@ -192,16 +199,8 @@
     };
 
     return function(obj, iteratee, memo, context) {
-      iteratee = optimizeCb(iteratee, context, 4);
-      var keys = !isArrayLike(obj) && _.keys(obj),
-          length = (keys || obj).length,
-          index = dir > 0 ? 0 : length - 1;
-      // Determine the initial value if none is provided.
-      if (arguments.length < 3) {
-        memo = obj[keys ? keys[index] : index];
-        index += dir;
-      }
-      return iterator(obj, iteratee, memo, keys, index, length);
+      var initial = arguments.length >= 3;
+      return reducer(obj, optimizeCb(iteratee, context, 4), memo, initial);
     };
   };