[LAA] Skip undef/poison strides in collectStridedAccess.
The map returned by collectStridedAccess is used to replace strides with
their versioned values. This does not work for Undef/Poison, which don't
have use-lists. Don't try to version them, as versioning won't be useful in
practice.
Fixes https://github.com/llvm/llvm-project/issues/162922.
