X64: Convert smis to holding 32 bits of payload.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 0295a99c3d91326d9aa1dcfd8ec2ee25cbc099fa..648e4869953bca2e38a23e64b694f75c9b52ead0 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -1679,8 +1679,8 @@ Object* Heap::AllocateConsString(String* first, String* second) {
       && second->IsAsciiRepresentation();
 
   // Make sure that an out of memory exception is thrown if the length
-  // of the new cons string is too large to fit in a Smi.
-  if (length > Smi::kMaxValue || length < -0) {
+  // of the new cons string is too large.
+  if (length > String::kMaxLength || length < 0) {
     Top::context()->mark_out_of_memory();
     return Failure::OutOfMemoryException();
   }