[mlir][linalg] Fix incorrect reduction detection in Vectorizer

When detecting reductions, make sure the block argument is from the linalg generic op.
This fixes https://github.com/iree-org/iree/issues/11779

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Co-authored-by: Andrzej Warzynski <andrzej.warzynski@arm.com>

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D141413