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

src/x64/lithium-codegen-x64.h

Index: src/x64/lithium-codegen-x64.h
===================================================================
--- src/x64/lithium-codegen-x64.h	(revision 7299)
+++ src/x64/lithium-codegen-x64.h	(working copy)
@@ -241,6 +241,11 @@
   // Caller should branch on equal condition.
   void EmitIsConstructCall(Register temp);
 
+  void EmitLoadField(Register result,
+                     Register object,
+                     Handle<Map> type,
+                     LookupResult* lookup);
+
   // Emits code for pushing a constant operand.
   void EmitPushConstantOperand(LOperand* operand);