Because -indvars widens induction variables through arithmetic,
`NeverNegative` cannot be a property of the `WidenIV` (a `WidenIV`
manages information for all transitive uses of an IV being widened,
including uses of `-1 * IV`). Instead it must live on `NarrowIVDefUse`
which manages information for a specific def-use edge in the transitive
use list of an induction variable.
This change also adds a test case that demonstrates the problem with
r248045.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248107 91177308-0d34-0410-b5e6-96231b3b80d8
8ba782a30c