[ScalarEvolution] Fold %x umin_seq %y if %x cannot be zero

Fold %x umin_seq %y to %x umin %y if %x cannot be zero. They only
differ in semantics for %x==0.

More generally %x *_seq %y folds to %x * %y if %x cannot be the
saturation fold (though currently we only have umin_seq).