[compiler] Remove broken support for undetectable strings.

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

Index: src/full-codegen/x64/full-codegen-x64.cc
diff --git a/src/full-codegen/x64/full-codegen-x64.cc b/src/full-codegen/x64/full-codegen-x64.cc
index ca73be24dd967e30f59c24315334eef38a0fd321..275d53aab9cc42524eab9384cadfab9d0461dc27 100644
--- a/src/full-codegen/x64/full-codegen-x64.cc
+++ b/src/full-codegen/x64/full-codegen-x64.cc
@@ -4981,12 +4981,8 @@ void FullCodeGenerator::EmitLiteralCompareTypeof(Expression* expr,
     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);