[mlir][sparse] made sparse vectorizer more robust on position of invariants (#80766)

Because the sparse vectorizer relies on the code coming out of the
sparsifier, the "patterns" are not always made very general. However, a
recent change in the generated code revealed an obvious situation where
the subscript analysis could be made a bit more robust.

Fixes:
https://github.com/llvm/llvm-project/issues/79897