Optionally use 31-bits SMI value for 64-bit system

src/x64/lithium-x64.h

Index: src/x64/lithium-x64.h
diff --git a/src/x64/lithium-x64.h b/src/x64/lithium-x64.h
index 50e32d8cd61ee999fd915aed6ddd4dc3ce0b3d07..f42b5dc9702f84c8a0d8448baa0114f14346ee6c 100644
--- a/src/x64/lithium-x64.h
+++ b/src/x64/lithium-x64.h
@@ -1926,6 +1926,7 @@ class LUint32ToDouble: public LTemplateInstruction<1, 1, 1> {
 };
 
 
+#if !V8_USE_31_BITS_SMI_VALUE
 class LNumberTagI: public LTemplateInstruction<1, 1, 0> {
  public:
   explicit LNumberTagI(LOperand* value) {
@@ -1936,6 +1937,20 @@ class LNumberTagI: public LTemplateInstruction<1, 1, 0> {
 
   DECLARE_CONCRETE_INSTRUCTION(NumberTagI, "number-tag-i")
 };
+#else
+class LNumberTagI: public LTemplateInstruction<1, 1, 1> {
+ public:
+  explicit LNumberTagI(LOperand* value, LOperand* temp) {
+    inputs_[0] = value;
+    temps_[0] = temp;
+  }
+
+  LOperand* value() { return inputs_[0]; }
+  LOperand* temp() { return temps_[0]; }
+
+  DECLARE_CONCRETE_INSTRUCTION(NumberTagI, "number-tag-i")
+};
+#endif
 
 
 class LNumberTagU: public LTemplateInstruction<1, 1, 1> {