Range_operator had a tree code added last release to facilitate bitmask 
operations.  This was intended to be a temporary change until we could 
figure out something more strategic going forward.

This patch removes the tree_code and replaces it with a virtual routine 
to perform the masking. Each of the affected tree codes operators now 
call the bitmask routine via a virtual function.  At some point we may 
want to consolidate the code that CCP is using so that it resides in the 
range_operator, but the extensive parameter list used by that CCP 
routine makes that prohibitive to do at the moment.

Bootstraps on x86_64-pc-linux-gnu with no regressions.  Pushed.

Andrew