Omit redundant smi checks in write barriers.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index ecc052291a471a30773d4e138ff14a6b0e0a5748..dafbd25122d75435617336ca28fa262a1c3f88f0 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -3866,6 +3866,8 @@ void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
 
   Register object = ToRegister(instr->object());
   Handle<Map> transition = instr->transition();
+  SmiCheck check_needed = hinstr->value()->IsHeapObject()
+                          ? OMIT_SMI_CHECK : INLINE_SMI_CHECK;
 
   if (FLAG_track_fields && representation.IsSmi()) {
     if (instr->value()->IsConstantOperand()) {
@@ -3886,6 +3888,9 @@ void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
         Register value = ToRegister(instr->value());
         Condition cc = masm()->CheckSmi(value);
         DeoptimizeIf(cc, 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()) {
@@ -3916,9 +3921,6 @@ void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
   }
 
   // Do the store.
-  SmiCheck check_needed = hinstr->value()->IsHeapObject()
-                          ? OMIT_SMI_CHECK : INLINE_SMI_CHECK;
-
   Register write_register = object;
   if (!access.IsInobject()) {
     write_register = ToRegister(instr->temp());