Ensure bv2nat and int2bv are not rewritten when using solve-bv-as-int (#6725)

Ensure bv2nat and int2bv are not rewritten when using solve-bv-as-int (#6725)

This PR ensures we do not eagerly rewrite bv2nat and int2bv when using solve-bv-as-int. Instead they are rewritten during expandDefinitions (at the end of preprocessing).

It also updates regressions that relied on lazy extended function reductions in the lazy solver to use solve-bv-as-int, and adds a missing case (INT_TO_BITVECTOR) in the solve-int-as-bv preprocessing pass.

A followup PR will remove support for lazy extended function reductions for bv2nat / int2bv altogether.