SDBM: fix the order of variables in the stripe-induced inequalities

The initial implementation of SDBM mistakenly swapped the order of variables in
the inequalities induced by a stripe equality: y = x # B actually implies
y - x <= 0 and x - y <= B - 1 rather than x - y <= 0 and y - x <= B - 1 as
implemented.  Textual comments in the test files were correct but did not
correspond to the emitted IR.  Round-tripping between SDBM and expression lists
was not affected because the wrong order was used in both directions of the
conversion.  Use the correct order.

PiperOrigin-RevId: 251252980