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

src/x64/lithium-x64.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 104 matching lines @@
   V(JSArrayLength)                              \
   V(Label)                                      \
   V(LazyBailout)                                \
   V(LoadContextSlot)                            \
   V(LoadElements)                               \
   V(LoadExternalArrayPointer)                   \
   V(LoadGlobal)                                 \
   V(LoadKeyedFastElement)                       \
   V(LoadKeyedGeneric)                           \
   V(LoadNamedField)                             \
+  V(LoadNamedFieldPolymorphic)                  \
   V(LoadNamedGeneric)                           \
   V(LoadFunctionPrototype)                      \
   V(LoadPixelArrayElement)                      \
   V(ModI)                                       \
   V(MulI)                                       \
   V(NumberTagD)                                 \
   V(NumberTagI)                                 \
   V(NumberUntagD)                               \
   V(ObjectLiteral)                              \
   V(OsrEntry)                                   \
@@ skipping 992 matching lines @@
  public:
   explicit LLoadNamedField(LOperand* object) {
     inputs_[0] = object;
   }
 
   DECLARE_CONCRETE_INSTRUCTION(LoadNamedField, "load-named-field")
   DECLARE_HYDROGEN_ACCESSOR(LoadNamedField)
 };
 
 
+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) {
     inputs_[0] = object;
   }
 
   DECLARE_CONCRETE_INSTRUCTION(LoadNamedGeneric, "load-named-generic")
   DECLARE_HYDROGEN_ACCESSOR(LoadNamedGeneric)
 
   LOperand* object() { return inputs_[0]; }
@@ skipping 950 matching lines @@
   DISALLOW_COPY_AND_ASSIGN(LChunkBuilder);
 };
 
 #undef DECLARE_HYDROGEN_ACCESSOR
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::int
 
 #endif  // V8_X64_LITHIUM_X64_H_