[compiler] Remove broken support for undetectable strings.

src/full-codegen/x64/full-codegen-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
@@ skipping 4963 matching lines @@
   PrepareForBailoutBeforeSplit(expr, true, if_true, if_false);
 
   Factory* factory = isolate()->factory();
   if (String::Equals(check, factory->number_string())) {
     __ JumpIfSmi(rax, if_true);
     __ movp(rax, FieldOperand(rax, HeapObject::kMapOffset));
     __ CompareRoot(rax, Heap::kHeapNumberMapRootIndex);
     Split(equal, if_true, if_false, fall_through);
   } else if (String::Equals(check, factory->string_string())) {
     __ JumpIfSmi(rax, if_false);
-    // Check for undetectable objects => false.
     __ CmpObjectType(rax, FIRST_NONSTRING_TYPE, rdx);
-    __ j(above_equal, if_false);
-    __ testb(FieldOperand(rdx, Map::kBitFieldOffset),
-             Immediate(1 << Map::kIsUndetectable));
-    Split(zero, if_true, if_false, fall_through);
+    Split(below, if_true, if_false, fall_through);
   } else if (String::Equals(check, factory->symbol_string())) {
     __ JumpIfSmi(rax, if_false);
     __ CmpObjectType(rax, SYMBOL_TYPE, rdx);
     Split(equal, if_true, if_false, fall_through);
   } else if (String::Equals(check, factory->boolean_string())) {
     __ CompareRoot(rax, Heap::kTrueValueRootIndex);
     __ j(equal, if_true);
     __ CompareRoot(rax, Heap::kFalseValueRootIndex);
     Split(equal, if_true, if_false, fall_through);
   } else if (String::Equals(check, factory->undefined_string())) {
@@ skipping 330 matching lines @@
             Assembler::target_address_at(call_target_address,
                                          unoptimized_code));
   return OSR_AFTER_STACK_CHECK;
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64