Set register values at the root, instead of at tips.
Make remove_computation( ) return the old computation.
Add new routine split_reg( ).
Only require call backedges if we have a result.
Fix copy_token( ).
Split regs before setting results.
Stop setting call back-edges when setting the call.
Remove no-longer-need code for preserving called_by in set_reg_value( ).
Fix split_reg( ) for if there is no call.
Remove make_terminal_token() and associated routines.