Make `--solve-int-as-bv=X` robust to rewriting (#6722)

Make `--solve-int-as-bv=X` robust to rewriting (#6722)

After commit 327a24508ed1d02a3fa233e680ffd0b30aa685a9, the int-to-bv
preprocessing pass is getting rewritten terms. As a result, the terms
can contain negative constants (instead of `(- c)`, i.e., `UMINUS` of a
positive constant) and `NONLINEAR_MULT`. The commit adds support for
those cases, does some minor cleanup, and adds regressions. The
regressions should allow us to detect if/when the preprocessing pass
breaks in the future.