MIPS: DummyUse if Representation::Tagged()+Type::Smi() to Representation::Smi()

Port r14802 (d4a53e02)

BUG=

Review URL: https://codereview.chromium.org/16019009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14811 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mips/lithium-mips.cc

@@ -1781,6 +1781,9 @@ LInstruction* LChunkBuilder::DoChange(HChange* instr) {
     } else if (to.IsSmi()) {
       HValue* val = instr->value();
       LOperand* value = UseRegister(val);
+      if (val->type().IsSmi()) {
+        return DefineSameAsFirst(new(zone()) LDummyUse(value));
+      }
       return AssignEnvironment(
           DefineSameAsFirst(new(zone()) LCheckSmiAndReturn(value)));
     } else {