deprecates exprt::move_to_operands(...)

Since sharing has been introduced there is no measureable performance
benefit when using move_to_operands compared to copying:

  for(int i=0; i<2000000; i++)
  {
    irep_idt id;
    symbol_exprt s(id, bool_typet());
    exprt something;
    something.move_to_operands(s);
    //something.copy_to_operands(s);
  }

with move_to_operands: 0.59s
with copy_to_operands: 0.55s

This does not appear to require a recent compiler.  Thus, there is no reason
to believe that moving ireps would be any faster than copying.  Moving has
the additional disadvantage that there is a side-effect on the object that
is moved.

A scenario in which moving offers a benefit is the idiom in which

1) an irep x is copied,
2) the copy is then subsequently modified,
3) x is then destroyed.

The proposal is to do step 1) via an rvalue reference and std::move()
instead.

Author: Daniel Kroening

src/util/expr.h

@@ -105,8 +105,13 @@ class exprt:public irept
   void reserve_operands(operandst::size_type n)
   { operands().reserve(n) ; }
 
+  DEPRECATED("use copy_to_operands(expr) instead")
   void move_to_operands(exprt &expr);
+
+  DEPRECATED("use copy_to_operands(e1, e2) instead")
   void move_to_operands(exprt &e1, exprt &e2);
+
+  DEPRECATED("use copy_to_operands(e1, e2, e3) instead")
   void move_to_operands(exprt &e1, exprt &e2, exprt &e3);
 
   /// Copy the given argument to the end of `exprt`'s operands.