bv: Assert input facts on user-level 0. (#6515)

bv: Assert input facts on user-level 0. (#6515)

The bitblast solver currently uses solving under assumptions for all facts that are sent to the bit-vector solver. For input facts on user-level 0 we can however assert the fact to the SAT solver, which allows the SAT solver to do more preprocessing. This PR adds the option to assert user-level 0 input facts, which is disabled by default.