[mlir][affine] Vectorizer test notifies the case with the unparallel loop with reduction

Vectorizer test does not support unparallel loop since no parallel reduction is available. We can emit the informational message instead of crashing the cli in such a case.
Fixes https://github.com/llvm/llvm-project/issues/61842

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D147765