[ValueTracking] Add logic for `isKnownNonZero(ssub.sat X, Y)`

The logic here is the same for normal `(sub X, Y)`, so just reused
`isNonZeroSub`.

Alive2 Link:
    https://alive2.llvm.org/ce/z/9kSkMv

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D149412