Introduce PushInt64AsTwoSmis and PopInt64AsTwoSmis macro instructions for X64

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 61abc206e108ec2f85e0ff83393eb778c58dde79..acc8f8b540fdacde79aadea749302e05b714ac56 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -720,6 +720,14 @@ class MacroAssembler: public Assembler {
   }
 
   void Push(Smi* smi);
+
+  // Save away a 64-bit integer on the stack as two 32-bit integers
+  // masquerading as smis so that the garbage collector skips visiting them.
+  void PushInt64AsTwoSmis(Register src, Register scratch = kScratchRegister);
+  // Reconstruct a 64-bit integer from two 32-bit integers masquerading as
+  // smis on the top of stack.
+  void PopInt64AsTwoSmis(Register dst, Register scratch = kScratchRegister);
+
   void Test(const Operand& dst, Smi* source);