[compiler] Remove broken support for undetectable strings.

src/ia32/lithium-codegen-ia32.cc

Index: src/ia32/lithium-codegen-ia32.cc
diff --git a/src/ia32/lithium-codegen-ia32.cc b/src/ia32/lithium-codegen-ia32.cc
index 9d8260da348f3ca9b6c6d67f7907678a2fcc1258..566b0a94c25368c06c5f77448487473c0c6e5705 100644
--- a/src/ia32/lithium-codegen-ia32.cc
+++ b/src/ia32/lithium-codegen-ia32.cc
@@ -5493,10 +5493,7 @@ Condition LCodeGen::EmitTypeofIs(LTypeofIsAndBranch* instr, Register input) {
   } else if (String::Equals(type_name, factory()->string_string())) {
     __ JumpIfSmi(input, false_label, false_distance);
     __ CmpObjectType(input, FIRST_NONSTRING_TYPE, input);
-    __ j(above_equal, false_label, false_distance);
-    __ test_b(FieldOperand(input, Map::kBitFieldOffset),
-              1 << Map::kIsUndetectable);
-    final_branch_condition = zero;
+    final_branch_condition = below;
 
   } else if (String::Equals(type_name, factory()->symbol_string())) {
     __ JumpIfSmi(input, false_label, false_distance);