MIPS: Omit redundant smi checks in write barriers.

Port r19323 (929c047)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/160713002

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19343 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mips/lithium-codegen-mips.cc

@@ -3967,12 +3967,18 @@ 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, scratch);
       DeoptimizeIf(eq, instr->environment(), scratch, Operand(zero_reg));
+
+      // 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());
@@ -4002,9 +4008,6 @@ 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);