[SelectionDAG] Remove redundant KnownBits smin and smax operations (#89519)

It turns out that if any of the operations can be zero, and neither of
the operands can be proven to be positive, it is possible for smax to be
zero, and KnownBits cannot prove otherwise even with KnownBits::smax. In
fact, proving it based on the KnownBits itself at that point without
increasing the depth is actually, provably impossible.

Same with smin.

This covers all the possible cases and is proven to be complete.