[ValueTracking] Add logic for `isKnownNonZero(umax X, Y)`

`(umax X, Y) != 0` -> `X != 0 || Y != 0`

Alive2 Link:
    https://alive2.llvm.org/ce/z/_Z9AUT

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D149415