Add "has fast elements" bit to maps and use it in inlined keyed loads.

src/x64/codegen-x64.cc

Index: src/x64/codegen-x64.cc
diff --git a/src/x64/codegen-x64.cc b/src/x64/codegen-x64.cc
index 572fa623eec145417e2c2ddd8c8cf4117100d176..3358b2cbae025f73a0cc64e670fddc7ea87556a7 100644
--- a/src/x64/codegen-x64.cc
+++ b/src/x64/codegen-x64.cc
@@ -7529,9 +7529,11 @@ Result CodeGenerator::EmitKeyedLoad() {
     // is not a dictionary.
     __ movq(elements.reg(),
             FieldOperand(receiver.reg(), JSObject::kElementsOffset));
-    __ Cmp(FieldOperand(elements.reg(), HeapObject::kMapOffset),
-           Factory::fixed_array_map());
-    deferred->Branch(not_equal);
+    if (FLAG_debug_code) {
+      __ Cmp(FieldOperand(elements.reg(), HeapObject::kMapOffset),
+             Factory::fixed_array_map());
+      __ Assert(equal, "JSObject with fast elements map has slow elements");
+    }
 
     // Check that key is within bounds.
     __ SmiCompare(key.reg(),