Revert trunk to version 3.16.4.

src/x64/code-stubs-x64.cc

Index: src/x64/code-stubs-x64.cc
diff --git a/src/x64/code-stubs-x64.cc b/src/x64/code-stubs-x64.cc
index 8fbc7dd70d903cf2fa350d80a47dabd5f3f0513c..3f17d03b92773ef035b037a006d7d651df76126a 100644
--- a/src/x64/code-stubs-x64.cc
+++ b/src/x64/code-stubs-x64.cc
@@ -4782,11 +4782,6 @@ void StringAddStub::Generate(MacroAssembler* masm) {
   // r9: second instance type.
   __ testb(rcx, Immediate(kAsciiDataHintMask));
   __ j(not_zero, &ascii_data);
-  __ xor_(r8, r9);
-  STATIC_ASSERT(kOneByteStringTag != 0 && kAsciiDataHintTag != 0);
-  __ andb(r8, Immediate(kOneByteStringTag | kAsciiDataHintTag));
-  __ cmpb(r8, Immediate(kOneByteStringTag | kAsciiDataHintTag));
-  __ j(equal, &ascii_data);
   // Allocate a two byte cons string.
   __ AllocateTwoByteConsString(rcx, rdi, no_reg, &call_runtime);
   __ jmp(&allocated);