[SMTChecker] Try Spacer's BMC engine for SAT cases

Currently we first run Spacer with preprocessing enabled (which doesn't give us counterexamples), and if it answers SAT, we run it again without preprocessing to get counterexamples. If the second answer is also SAT, great, we get counterexamples. In case the problem without preprocessing is too hard, we use the first answer (SAT) without counterexamples. In the latter case, we can also try Spacer's BMC engine which could be better when handling SAT cases.

Start by tracing the SMTChecker path that runs Spacer with preprocessing, retries without preprocessing for counterexamples, and handles SAT results. Determine where Spacer's BMC engine can be tried when the second run is too hard; done means SAT cases use that fallback when applicable and preserve the available counterexample behavior.

Written by the indexing model from the issue text.