Do not lookup pathed helpers on the helper stack

Fixes #764

lib/handlebars/compiler/javascript-compiler.js

@@ -369,7 +369,7 @@ JavaScriptCompiler.prototype = {
           return ' != null ? ' + lookup + ' : ' + current;
         } else {
           // Otherwise we can use generic falsy handling
-          return ' != null && ' + lookup;
+          return (falsy ? ' && ' : ' != null && ') + lookup;
         }
       }, true);
     }
@@ -527,19 +527,18 @@ JavaScriptCompiler.prototype = {
   // and pushes the helper's return value onto the stack.
   //
   // If the helper is not found, `helperMissing` is called.
-  invokeHelper: function(paramSize, name, isRoot) {
+  invokeHelper: function(paramSize, name, isSimple, isRoot) {
     this.aliases.helperMissing = 'helpers.helperMissing';
-    this.useRegister('helper');
 
     var nonHelper = this.popStack();
     var helper = this.setupHelper(paramSize, name);
 
-    var lookup = 'helper = ' + helper.name + ' || ' + nonHelper + ' || helperMissing';
+    var lookup = (isSimple ? helper.name + ' || ' : '') + nonHelper + ' || helperMissing';
     if (helper.paramsInit) {
       lookup += ',' + helper.paramsInit;
     }
 
-    this.push('(' + lookup + ',helper.call(' + helper.callParams + '))');
+    this.push('((' + lookup + ').call(' + helper.callParams + '))');
 
     // Always flush subexpressions. This is both to prevent the compounding size issue that
     // occurs when the code has to be duplicated for inlining and also to prevent errors