Also make a slight change to ir_variable. The ir_dereference tracks
the number of nested dereferences. If an ir_variable is visited and
the count is non-zero, just print the name of the variable.
The ir_visitor class is the abstract base class for all visitors.
ir_print_visitor contains the beginnings of a concrete visitor class
that will print out an IR sequence in a Lisp / Scheme-like syntax.
