x64: Make sure that the upper half of a 64bit register contains 0 for int32 values.

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 1c8bb0d381d23f749849636136134ef03500c6aa..43a676c8b49241dbe8183483328b33b3c73e11dd 100644
--- a/src/x64/lithium-x64.h
+++ b/src/x64/lithium-x64.h
@@ -1246,6 +1246,9 @@ class LBitI V8_FINAL : public LTemplateInstruction<1, 2, 0> {
   LOperand* right() { return inputs_[1]; }
 
   Token::Value op() const { return hydrogen()->op(); }
+  bool IsInteger32() const {
+    return hydrogen()->representation().IsInteger32();
+  }
 
   DECLARE_CONCRETE_INSTRUCTION(BitI, "bit-i")
   DECLARE_HYDROGEN_ACCESSOR(Bitwise)