This bug caused the expression x + (-2) < -3 to be rewritten as
(x > 2) || (x < -1), instead of (x > 2) && (x < -1).
We had tests for a very similar case, but this specific example was
triggered by having the two constant terms in the inequality with a
difference of only one.
This commit rewrites and unifies the logic for handling inequalities,
and adapts the test for checking this specific additional corner case.