Factor out a commonly used code sequence to DropAndMove(Location, Register)

src/fast-codegen.h

Index: src/fast-codegen.h
===================================================================
--- src/fast-codegen.h	(revision 3152)
+++ src/fast-codegen.h	(working copy)
@@ -60,6 +60,10 @@
   void Move(Register destination, Location source);
   void Move(Slot* destination, Location source);
 
+  // Drop the TOS, and store source to destination.
+  // If destination is TOS, just overwrite TOS with source.
+  void DropAndMove(Location destination, Register source);
+
   void VisitDeclarations(ZoneList<Declaration*>* declarations);
   Handle<JSFunction> BuildBoilerplate(FunctionLiteral* fun);
   void DeclareGlobals(Handle<FixedArray> pairs);