Allow scalable vectorization of linalg::reduce and linalg::generic that has
reduction iterator(s) with two restrictions:
1. The reduction dim is the last (innermost) dim of the op; and
2. Only the reduction dim is requested for scalable vectorization.
One exception is that scalable vectorization of the reduction dim in
Matmul-like ops are not supported even above restrictions are met.
Allowed combinations of scalable flags and iterator types:
Matmul:
Iterators: ["parallel", "parallel", "reduction"]
Scalable Flags: ["true", "true", "false"]
["false", "true", "false"]
Matvec:
Iterators: ["parallel", "reduction"]
Scalable Flags: ["false", "true"]
["true", "false"]
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