FM: collect implied formulae in & arguments; do not to translate them
* src/tgbaalgos/ltl2tgba_fm.cc (implied_subforfmulae): New function. (ltl_trad_visitor::visit(const binop*)): Use it. This is an attempt to correct the unoptimal translation of 'Fa & GFa' left by previous patch. It still fails to optimize 'Fa & GF(a&b)', but this is not a regression compared to previous version.