[compiler] Remove broken support for undetectable strings.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index 428aaa64b6927c2f5f5e5015abe6ddbdefe2a9be..84eabaf1f3d891958d6fc3dcf228bc0122960786 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -5697,10 +5697,7 @@ 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);
-    __ testb(FieldOperand(input, Map::kBitFieldOffset),
-             Immediate(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);