[ValueTracking] Don't require non-zero step for add nuw

It's okay if the step is zero, we'll just stay at the same non-zero
value in that case. The valuable part of this is that the step
doesn't even need to be a constant anymore.

GitOrigin-RevId: 5c0fb026c93b33dd107f6ff50723d7eed911e1ce
2 files changed