Omit redundant smi checks in write barriers.

src/ia32/lithium-codegen-ia32.cc

Index: src/ia32/lithium-codegen-ia32.cc
diff --git a/src/ia32/lithium-codegen-ia32.cc b/src/ia32/lithium-codegen-ia32.cc
index bcb90ca802677f0420aa5f0956f7292a5daa09fe..18ef79a629c2d5420569a333c6ffbe9af1d041eb 100644
--- a/src/ia32/lithium-codegen-ia32.cc
+++ b/src/ia32/lithium-codegen-ia32.cc
@@ -4289,6 +4289,9 @@ void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
 
   Register object = ToRegister(instr->object());
   Handle<Map> transition = instr->transition();
+  SmiCheck check_needed =
+      instr->hydrogen()->value()->IsHeapObject()
+          ? OMIT_SMI_CHECK : INLINE_SMI_CHECK;
 
   if (FLAG_track_fields && representation.IsSmi()) {
     if (instr->value()->IsConstantOperand()) {
@@ -4308,6 +4311,9 @@ void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
         Register value = ToRegister(instr->value());
         __ test(value, Immediate(kSmiTagMask));
         DeoptimizeIf(zero, 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()) {
@@ -4345,10 +4351,6 @@ void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
   }
 
   // Do the store.
-  SmiCheck check_needed =
-      instr->hydrogen()->value()->IsHeapObject()
-          ? OMIT_SMI_CHECK : INLINE_SMI_CHECK;
-
   Register write_register = object;
   if (!access.IsInobject()) {
     write_register = ToRegister(instr->temp());