X64: Made simpler version of fast-smi-loop code.

src/x64/virtual-frame-x64.h

Index: src/x64/virtual-frame-x64.h
diff --git a/src/x64/virtual-frame-x64.h b/src/x64/virtual-frame-x64.h
index 0549e3cc2a4bbe338995c103500249b0d63623b9..adf47e21678593f5f3fd8843b9308ee8a7f40260 100644
--- a/src/x64/virtual-frame-x64.h
+++ b/src/x64/virtual-frame-x64.h
@@ -388,6 +388,13 @@ class VirtualFrame : public ZoneObject {
   // Duplicate the top element of the frame.
   void Dup() { PushFrameSlotAt(element_count() - 1); }
 
+  // Duplicate the n'th element from the top of the frame.
+  // Dup(1) is equivalent to Dup().
+  void Dup(int n) {
+    ASSERT(n > 0);
+    PushFrameSlotAt(element_count() - n);
+  }
+
   // Pop an element from the top of the expression stack.  Returns a
   // Result, which may be a constant or a register.
   Result Pop();