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

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
===================================================================
--- src/x64/lithium-x64.cc	(revision 7299)
+++ src/x64/lithium-x64.cc	(working copy)
@@ -1763,6 +1763,21 @@
 }
 
 
+LInstruction* LChunkBuilder::DoLoadNamedFieldPolymorphic(
+    HLoadNamedFieldPolymorphic* instr) {
+  ASSERT(instr->representation().IsTagged());
+  if (instr->need_generic()) {
+    LOperand* obj = UseFixed(instr->object(), rax);
+    LLoadNamedFieldPolymorphic* result = new LLoadNamedFieldPolymorphic(obj);
+    return MarkAsCall(DefineFixed(result, rax), instr);
+  } else {
+    LOperand* obj = UseRegisterAtStart(instr->object());
+    LLoadNamedFieldPolymorphic* result = new LLoadNamedFieldPolymorphic(obj);
+    return AssignEnvironment(DefineAsRegister(result));
+  }
+}
+
+
 LInstruction* LChunkBuilder::DoLoadNamedGeneric(HLoadNamedGeneric* instr) {
   LOperand* object = UseFixed(instr->object(), rax);
   LLoadNamedGeneric* result = new LLoadNamedGeneric(object);