X87: [compiler] Remove broken support for undetectable strings.

src/x87/lithium-codegen-x87.cc

Index: src/x87/lithium-codegen-x87.cc
diff --git a/src/x87/lithium-codegen-x87.cc b/src/x87/lithium-codegen-x87.cc
index 00c0d2d8166c6e20668897deb8e509466be1d1db..9328ff664efa915dc207efb714d8612e1565c8f4 100644
--- a/src/x87/lithium-codegen-x87.cc
+++ b/src/x87/lithium-codegen-x87.cc
@@ -6108,10 +6108,7 @@ 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);