[date] Refactor the %_DateField intrinsic to be optimizable.

src/mips/lithium-codegen-mips.cc

Index: src/mips/lithium-codegen-mips.cc
diff --git a/src/mips/lithium-codegen-mips.cc b/src/mips/lithium-codegen-mips.cc
index 910c83a82d1ae30019dd36e5560ab66620b19fd4..2310895bb6c47ef27189fcd374f6ac472ad85b30 100644
--- a/src/mips/lithium-codegen-mips.cc
+++ b/src/mips/lithium-codegen-mips.cc
@@ -1800,12 +1800,6 @@ void LCodeGen::DoDateField(LDateField* instr) {
   DCHECK(!scratch.is(scratch0()));
   DCHECK(!scratch.is(object));
 
-  __ SmiTst(object, at);
-  DeoptimizeIf(eq, instr, Deoptimizer::kSmi, at, Operand(zero_reg));
-  __ GetObjectType(object, scratch, scratch);
-  DeoptimizeIf(ne, instr, Deoptimizer::kNotADateObject, scratch,
-               Operand(JS_DATE_TYPE));
-
   if (index->value() == 0) {
     __ lw(result, FieldMemOperand(object, JSDate::kValueOffset));
   } else {