[kernel] Do not restrict cast to unsigned expressions for downcasts

src/kernel_services/ast_data/alarms.ml

tests/rte/oracle/bts2314.res.oracle

tests/rte/oracle/divmod.res.oracle

tests/rte/oracle/divmod_typedef.res.oracle

tests/rte/oracle/downcast.0.res.oracle

tests/rte/oracle/downcast.2.res.oracle

tests/rte/oracle/minus.0.res.oracle

tests/rte/oracle/minus.1.res.oracle

tests/rte_manual/oracle/signed_downcast.1.res.oracle

tests/rte_manual/oracle/unsigned_downcast.res.oracle

tests/value/oracle/downcast.res.oracle