[DA] Update stale weak-crossing SIV FIXMEs (NFC) (#206941)
Remove outdated FIXME comments from weak-crossing SIV tests where the
expected dependence results are now documented by the CHECK lines.
Also clarify the overflow test comment and fix the described second
access from A[3*i - 2] to A[3*i + 1].
Signed-off-by: Ruoyu Qiu <cabbaken@outlook.com>
