Omit redundant smi checks in write barriers.

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index dde402303aef709c0f4b06e43003bff22448eb22..54ec58988cfe53ce34907147369a94567bca6eff 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -4051,12 +4051,18 @@ void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
   }
 
   Handle<Map> transition = instr->transition();
+  SmiCheck check_needed =
+      instr->hydrogen()->value()->IsHeapObject()
+          ? OMIT_SMI_CHECK : INLINE_SMI_CHECK;
 
   if (FLAG_track_heap_object_fields && representation.IsHeapObject()) {
     Register value = ToRegister(instr->value());
     if (!instr->hydrogen()->value()->type().IsHeapObject()) {
       __ SmiTst(value);
       DeoptimizeIf(eq, instr->environment());
+
+      // We know that value is a smi now, so we can omit the check below.
+      check_needed = OMIT_SMI_CHECK;
     }
   } else if (representation.IsDouble()) {
     ASSERT(transition.is_null());
@@ -4086,9 +4092,6 @@ void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
 
   // Do the store.
   Register value = ToRegister(instr->value());
-  SmiCheck check_needed =
-      instr->hydrogen()->value()->IsHeapObject()
-          ? OMIT_SMI_CHECK : INLINE_SMI_CHECK;
   if (access.IsInobject()) {
     MemOperand operand = FieldMemOperand(object, offset);
     __ Store(value, operand, representation);