Improve Array.shift() performance for small arrays.

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 093b95b4dd012b29854756a32a54400b4404c383..86121f62a333fed565d6ecd668af5cde83a7699d 100644
--- a/src/x64/lithium-x64.h
+++ b/src/x64/lithium-x64.h
@@ -26,6 +26,7 @@ class LCodeGen;
   V(ArgumentsLength)                            \
   V(ArithmeticD)                                \
   V(ArithmeticT)                                \
+  V(ArrayShift)                                 \
   V(BitI)                                       \
   V(BoundsCheck)                                \
   V(Branch)                                     \
@@ -2245,6 +2246,21 @@ class LTransitionElementsKind V8_FINAL : public LTemplateInstruction<0, 2, 2> {
 };
 
 
+class LArrayShift V8_FINAL : public LTemplateInstruction<1, 2, 0> {
+ public:
+  LArrayShift(LOperand* context, LOperand* object) {
+    inputs_[0] = context;
+    inputs_[1] = object;
+  }
+
+  LOperand* context() const { return inputs_[0]; }
+  LOperand* object() const { return inputs_[1]; }
+
+  DECLARE_CONCRETE_INSTRUCTION(ArrayShift, "array-shift")
+  DECLARE_HYDROGEN_ACCESSOR(ArrayShift)
+};
+
+
 class LTrapAllocationMemento V8_FINAL : public LTemplateInstruction<0, 1, 1> {
  public:
   LTrapAllocationMemento(LOperand* object,