[SelectionDAG] Add/Improve cases in `isKnownNeverZero`

1) Handle casts a bit more cleanly just with a loop rather than with
   recursion.

2) Add additional cases for smin/smax

3 ) For shifts we can also deduce non-zero if the maximum shift amount
    on the known 1s is non-zero.

Differential Revision: https://reviews.llvm.org/D156777