[BasicAA] Handle known non-zero variable index

BasicAA currently handles cases like Scale*V0 + (-Scale)*V1 where
V0 != V1, but does not handle the simpler case of Scale*V with
V != 0. Add it based on an isKnownNonZero() call.

I'm not passing a context instruction for now, because the existing
approach of always using GEP1 for context could result in symmetry
issues.

Differential Revision: https://reviews.llvm.org/D93162