int-to-bv: correct model values (#6811)

int-to-bv: correct model values (#6811)

the int-to-bv preprocessing pass produced wrong models.
This PR fixes this in a similar fashion to other preprocessing passes, by adding a substitution to the preprocessing pass context. This requires moving the main translation function to be a class method, rather than a helper method in an empty namespace.

Thanks to @alex-ozdemir for raising this issue and producing a triggering benchmark (added to regressions in this PR).