reduce_visitor::visit(binop).

* src/ltlvisit/reduce.cc (eventual_universal_visitor::recurse_):
Move this method...
(recurse_eu): ... outside as a separate function.  Likewise for
the universal/eventual result struct.
(reduce_visitor::visit(binop)): Call recurse_eu() once to replace
two calls to is_eventual and is_universal, thus replacing two
recursions by one.