Enable GVN for polymorphic loads by not expanding them at the HIR level.

src/x64/lithium-x64.h

Index: src/x64/lithium-x64.h
===================================================================
--- src/x64/lithium-x64.h	(revision 7299)
+++ src/x64/lithium-x64.h	(working copy)
@@ -122,6 +122,7 @@
   V(LoadKeyedFastElement)                       \
   V(LoadKeyedGeneric)                           \
   V(LoadNamedField)                             \
+  V(LoadNamedFieldPolymorphic)                  \
   V(LoadNamedGeneric)                           \
   V(LoadFunctionPrototype)                      \
   V(LoadPixelArrayElement)                      \
@@ -1134,6 +1135,19 @@
 };
 
 
+class LLoadNamedFieldPolymorphic: public LTemplateInstruction<1, 1, 0> {
+ public:
+  explicit LLoadNamedFieldPolymorphic(LOperand* object) {
+    inputs_[0] = object;
+  }
+
+  DECLARE_CONCRETE_INSTRUCTION(LoadNamedField, "load-named-field-polymorphic")
+  DECLARE_HYDROGEN_ACCESSOR(LoadNamedFieldPolymorphic)
+
+  LOperand* object() { return inputs_[0]; }
+};
+
+
 class LLoadNamedGeneric: public LTemplateInstruction<1, 1, 0> {
  public:
   explicit LLoadNamedGeneric(LOperand* object) {