[NFC][InstCombine] Add test coverage for @llvm.uadd.sat canonicalization

The non-strict variants are already handled because they are canonicalized
to strict variants by swapping hands in both the select and icmp,
and the fold simply considers that strictness is irrelevant here.

But that isn't actually true for the last pattern, as PR48390 reports.

(cherry picked from commit f16320b9)