Fix rewriting of mixed-integer atoms (#8214)

Fix rewriting of mixed-integer atoms (#8214)

The sixth input from #8159 revealed a subtle issue in our rewriting of arithmetic atoms. As real and integer atoms are rewritten differently, we check this and then call different functions. However, we check this on the original input, although this property can change during rewriting, e.g., when the only real variable vanishes.
This PR fixes this issue.