[x64] Improve key value sign-extension of dehoisted LoadKeyed/StoreKeyed

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 a58208c816f4f0471be89a44f6e6374b2bc0acdc..becd862ded5c50c8b58283419657ac17912f07bd 100644
--- a/src/x64/lithium-x64.h
+++ b/src/x64/lithium-x64.h
@@ -266,6 +266,8 @@ class LInstruction : public ZoneObject {
 
   virtual bool HasInterestingComment(LCodeGen* gen) const { return true; }
 
+  virtual bool MustSignExtendResult() const { return false; }
+
 #ifdef DEBUG
   void VerifyCall();
 #endif
@@ -301,6 +303,8 @@ class LTemplateResultInstruction : public LInstruction {
   void set_result(LOperand* operand) { results_[0] = operand; }
   LOperand* result() const { return results_[0]; }
 
+  virtual bool MustSignExtendResult() const V8_FINAL V8_OVERRIDE;
+
  protected:
   EmbeddedContainer<LOperand*, R> results_;
 };