[ValueTracking] Use `KnownBits` functions for `computeKnownBits` of saturating add/sub functions

The knownbits implementation covers all the cases previously handled
by `uadd.sat`/`usub.sat` as well some additional ones. We previously
were not handling the `ssub.sat`/`sadd.sat` cases at all.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D150103