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

src/arm/lithium-arm.cc

Index: src/arm/lithium-arm.cc
===================================================================
--- src/arm/lithium-arm.cc	(revision 7299)
+++ src/arm/lithium-arm.cc	(working copy)
@@ -1773,6 +1773,21 @@
 }
 
 
+LInstruction* LChunkBuilder::DoLoadNamedFieldPolymorphic(
+    HLoadNamedFieldPolymorphic* instr) {
+  ASSERT(instr->representation().IsTagged());
+  if (instr->need_generic()) {
+    LOperand* obj = UseFixed(instr->object(), r0);
+    LLoadNamedFieldPolymorphic* result = new LLoadNamedFieldPolymorphic(obj);
+    return MarkAsCall(DefineFixed(result, r0), 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(), r0);
   LInstruction* result = DefineFixed(new LLoadNamedGeneric(object), r0);