Simplify F(a)|F(b) as F(a|b). Add similar rule for G(a)&G(b).
* src/ltlvisit/basicreduce.cc (basic_reduce_visitor): Replace the FG(a)|FG(b) == FG(a|b) rule by the above more generic one. Add the dual rule for G(a)&G(b), as we had none (this one won't improve anything in the translation, but it is more symmetric this way). Also simplify some pointer checks.