Thiese commits add three more populate methods for
`vector.multi_reduction`'s lowering patterns:
* populateVectorMultiReductionTransformationPatterns
* populateVectorMultiReductionFlatteningPatterns
* populateVectorMultiReductionUnrollingPatterns
These methods have a
finer level of granularity and allow users to select between unrolling,
flattening, and applying transformations that would set up operations
for unrolling and flattening.
The previous populateVectorMultiReductionLoweringPatterns method
is rewritten in terms of these new methods.
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