In MacroAssembler::JumpIfNotBothSequentialAsciiStrings a custom mask

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index b3e15905aad4654d41ed8041bac60424694959fb..ddd0f978c05d7fd6fa6a6f0f7507de1ce21655fa 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -2253,7 +2253,11 @@ void MacroAssembler::JumpIfNotBothSequentialAsciiStrings(
   ASSERT(kNotStringTag != 0);
   const int kFlatAsciiStringMask =
       kIsNotStringMask | kStringRepresentationMask | kStringEncodingMask;
-  const int kFlatAsciiStringTag = ASCII_STRING_TYPE;
+  // Use ASCII_INTERNALIZED_STRING_TYPE because it's internalized bit is zero.
+  // Since we don't include that bit in the mask above, the test below will
+  // succeed for ASCII_STRING_TYPE and ASCII_INTERNALIZED_STRING_TYPE.
+  STATIC_ASSERT(kInternalizedTag == 0);
+  const int kFlatAsciiStringTag = ASCII_INTERNALIZED_STRING_TYPE;
 
   andl(scratch1, Immediate(kFlatAsciiStringMask));
   andl(scratch2, Immediate(kFlatAsciiStringMask));