[DA] Consolidate the logic for checking overlap at the boundary (NFCI) (#189341)
In the Weak Crossing SIV test, there were two places where we checked
the dependency at the boundary, one is at the first iteration, and the
other is at the last iteration. Now the former can be merged into the
latter. There used to be an edge case when the coefficient is zero, and
we had an explicit check for that. This patch removes that check as
well, by moving the boundary check after the assertion that ensures the
(maybe negated) coefficient is positive.