Use weak cells in map checks in polymorphic ICs.

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 d8878a83ee3890a5fa181c0c4124b2e195a6912c..80fc13a2ebcb81d5eaf1a81c8bd4f76faf9f6962 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -2852,6 +2852,13 @@ void MacroAssembler::LoadGlobalCell(Register dst, Handle<Cell> cell) {
 }
 
 
+void MacroAssembler::CmpWeakValue(Register value, Register scratch,
+                                  Handle<WeakCell> cell) {
+  Move(scratch, cell, RelocInfo::EMBEDDED_OBJECT);
+  cmpp(value, FieldOperand(scratch, WeakCell::kValueOffset));
+}
+
+
 void MacroAssembler::Drop(int stack_elements) {
   if (stack_elements > 0) {
     addp(rsp, Immediate(stack_elements * kPointerSize));