Add complete ElementKind information directly to Map for objects with elements.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index 329fd18dc47cbbde28cd7a16e274d84159cda4e1..512a4553e5a330f6edcc6e489b2606e839cce157 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -3330,9 +3330,7 @@ void FullCodeGenerator::EmitFastAsciiArrayJoin(ZoneList<Expression*>* args) {
   __ j(not_equal, &bailout);
 
   // Check that the array has fast elements.
-  __ testb(FieldOperand(scratch, Map::kBitField2Offset),
-           Immediate(1 << Map::kHasFastElements));
-  __ j(zero, &bailout);
+  __ CheckFastElements(scratch, &bailout);
 
   // Array has fast elements, so its length must be a smi.
   // If the array has length zero, return the empty string.