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

src/arm/lithium-codegen-arm.h

Index: src/arm/lithium-codegen-arm.h
===================================================================
--- src/arm/lithium-codegen-arm.h	(revision 7299)
+++ src/arm/lithium-codegen-arm.h	(working copy)
@@ -264,6 +264,11 @@
   // Caller should branch on equal condition.
   void EmitIsConstructCall(Register temp1, Register temp2);
 
+  void EmitLoadField(Register result,
+                     Register object,
+                     Handle<Map> type,
+                     LookupResult* lookup);
+
   LChunk* const chunk_;
   MacroAssembler* const masm_;
   CompilationInfo* const info_;