Use weak cells in map checks in polymorphic ICs.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 24eea383949527c3edb7df1e37f34a032884afe3..4d36842e9dee22be0641e8124b69f119503e2044 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -847,6 +847,8 @@ class MacroAssembler: public Assembler {
   // Load a global cell into a register.
   void LoadGlobalCell(Register dst, Handle<Cell> cell);
 
+  void CmpWeakValue(Register value, Register scratch, Handle<WeakCell> cell);
+
   // Emit code to discard a non-negative number of pointer-sized elements
   // from the stack, clobbering only the rsp register.
   void Drop(int stack_elements);