[BVMiniSat] Avoid duplicates in conflicts (#1745)

[BVMiniSat] Avoid duplicates in conflicts (#1745)

If analyzeFinal() in BVMiniSat was called with a literal that also
occurred in the trail, it could happen that the set of assumptions that
led to the assignment of `p` contained `p` twice.
BitVectorProof::endBVConflict would then register that conflict with the
duplicate literal but at the same time compute a conflict expression
with the duplicate removed. LFSCSatProof::printAssumptionsResolution
would then output two resolutions for the same literal, which made the
simplify_clause side condition fail because it couldn't find the literal
in the clause anymore after the first removal.

The fix simply avoid adding `p` to the set of assumptions if it is found
in the trail. In this situation, `p` is guaranteed to be in the set of
assumptions already because it has been added at the beginning of
analyzeFinal(). This issue originally came up in PR #1384. With this fix
the regression tests pass in #1384.