Simplify prototype chain walk in hydrogen

Since we know that we're only encountering JSObjects, we can use the
faster GetPrototype() method to walk up the chain

BUG=none
R=verwaest@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/321543002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21718 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen.cc

@@ -6558,11 +6558,10 @@ HInstruction* HOptimizedGraphBuilder::BuildMonomorphicElementAccess(
     // changes could allow callbacks on elements in the chain that
     // aren't compatible with monomorphic keyed stores.
     Handle<JSObject> prototype(JSObject::cast(map->prototype()));
-    Object* holder = map->prototype();
-    while (holder->GetPrototype(isolate())->IsJSObject()) {
-      holder = holder->GetPrototype(isolate());
+    JSObject* holder = JSObject::cast(map->prototype());
+    while (!holder->GetPrototype()->IsNull()) {
+      holder = JSObject::cast(holder->GetPrototype());
     }
-    ASSERT(holder->GetPrototype(isolate())->IsNull());
 
     BuildCheckPrototypeMaps(prototype,
                             Handle<JSObject>(JSObject::cast(holder)));