[compiler] Introduce StringEqualStub and StringNotEqualStub.

test/cctest/compiler/test-simplified-lowering.cc

Index: test/cctest/compiler/test-simplified-lowering.cc
diff --git a/test/cctest/compiler/test-simplified-lowering.cc b/test/cctest/compiler/test-simplified-lowering.cc
index 1b752edd3c855138d5f909b38a0308528c53aea6..dc790ff98aea27d583c980adc7d4ac31b2979a22 100644
--- a/test/cctest/compiler/test-simplified-lowering.cc
+++ b/test/cctest/compiler/test-simplified-lowering.cc
@@ -1144,13 +1144,10 @@ TEST(LowerReferenceEqual_to_wordeq) {
 TEST(LowerStringOps_to_call_and_compare) {
     // These tests need linkage for the calls.
     TestingGraph t(Type::String(), Type::String());
-    IrOpcode::Value compare_eq =
-        static_cast<IrOpcode::Value>(t.machine()->WordEqual()->opcode());
     IrOpcode::Value compare_lt =
         static_cast<IrOpcode::Value>(t.machine()->IntLessThan()->opcode());
     IrOpcode::Value compare_le = static_cast<IrOpcode::Value>(
         t.machine()->IntLessThanOrEqual()->opcode());
-    t.CheckLoweringStringBinop(compare_eq, t.simplified()->StringEqual());
     t.CheckLoweringStringBinop(compare_lt, t.simplified()->StringLessThan());
     t.CheckLoweringStringBinop(compare_le,
                                t.simplified()->StringLessThanOrEqual());