Rewrite xor, =>, and <=> in negative_normal_form().
* src/ltlvisit/simplify.hh, src/ltlvisit/simplify.cc (ltl_simplify::negative_normal_form): Remove the third parameter and always rewrite XOR, =>, and <=>. This avoid problems with the cache, that could have been populated with a different value for this third parameter. * src/ltltest/reduc.cc, src/tgbaalgos/ltl2tgba_fm.cc: Adjust calls to negative_normal_form(). * src/ltltest/nenoform.test: Adjust tests.