MIPS: Assign less environments.

Port r14983 (960d6de)

BUG=

Review URL: https://codereview.chromium.org/16567002
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14988 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mips/lithium-mips.cc

@@ -2229,10 +2229,11 @@ LInstruction* LChunkBuilder::DoStoreNamedField(HStoreNamedField* instr) {
   LOperand* temp = needs_write_barrier_for_map ? TempRegister() : NULL;
 
   LStoreNamedField* result = new(zone()) LStoreNamedField(obj, val, temp);
-  if ((FLAG_track_fields && instr->field_representation().IsSmi()) ||
-      (FLAG_track_heap_object_fields &&
-       instr->field_representation().IsHeapObject())) {
-    return AssignEnvironment(result);
+  if (FLAG_track_heap_object_fields &&
+      instr->field_representation().IsHeapObject()) {
+    if (!instr->value()->type().IsHeapObject()) {
+      return AssignEnvironment(result);
+    }
   }
   return result;
 }