Port r7868 (constant masking) to 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 1c1cd95e94d8e4d93a1594947a817813b7503a28..68795a48c1f4e6ca73c282cd8983d9ae290cb775 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -774,6 +774,11 @@ class MacroAssembler: public Assembler {
   // Move if the registers are not identical.
   void Move(Register target, Register source);
 
+  // Support for constant splitting.
+  bool IsUnsafeInt(const int x);
+  void SafeMove(Register dst, Smi* src);
+  void SafePush(Smi* src);
+
   // Bit-field support.
   void TestBit(const Operand& dst, int bit_index);