Implement basic array prefetching hints in Hydrogen.

src/code-stubs-hydrogen.cc

Index: src/code-stubs-hydrogen.cc
===================================================================
--- src/code-stubs-hydrogen.cc	(revision 13350)
+++ src/code-stubs-hydrogen.cc	(working copy)
@@ -82,6 +82,11 @@
   context_ = new(zone()) HContext();
   AddInstruction(context_);
 
+  HConstant* undefined_constant = new(zone()) HConstant(
+      info_.isolate()->factory()->undefined_value(), Representation::Tagged());
+  AddInstruction(undefined_constant);
+  graph()->set_undefined_constant(undefined_constant);
+
   int major_key = stub()->MajorKey();
   CodeStubInterfaceDescriptor* descriptor =
       info_.isolate()->code_stub_interface_descriptor(major_key);