get_subexpression_at_offset: use typecast_exprt for pointer-to-pointer casts

This signals that the cast is cleaner than a general byte-extract a little more clearly

Author: smowton

src/util/pointer_offset_size.cpp

@@ -655,6 +655,13 @@ exprt get_subexpression_at_offset(
     return result;
   }
 
+  if(
+    offset_bytes == 0 && expr.type().id() == ID_pointer &&
+    target_type_raw.id() == ID_pointer)
+  {
+    return typecast_exprt(expr, target_type_raw);
+  }
+
   const typet &source_type = ns.follow(expr.type());
   const auto target_size_bits = pointer_offset_bits(target_type_raw, ns);
   if(!target_size_bits.has_value())