Reland parts of '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..44317458e2c47d0763cfc9f1ee4ea5caa4224654 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -847,6 +847,13 @@ class MacroAssembler: public Assembler {
   // Load a global cell into a register.
   void LoadGlobalCell(Register dst, Handle<Cell> cell);
 
+  // Compare the given value and the value of weak cell.
+  void CmpWeakValue(Register value, Handle<WeakCell> cell, Register scratch);
+
+  // Load the value of the weak cell in the value register. Branch to the given
+  // miss label if the weak cell was cleared.
+  void LoadWeakValue(Register value, Handle<WeakCell> cell, Label* miss);
+
   // 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);