GF(a|Xb) = GF(a|b)      GF(a|Fb) = GF(a|b)
FG(a&Xb) = FG(a&b)      FG(a&Gb) = FG(a&b)

* src/ltlvisit/simplify.cc: Implement them.
* NEWS, doc/tl/tl.tex: Document them.
* src/ltltest/reduccmp.test: Test then.