[mlir][vector] Refactor multi-reduction patterns (NFC) (#183048)
Refactor the following patterns to inherit from
`MaskableOpRewritePattern`:
* `TwoDimMultiReductionToReduction`
* `TwoDimMultiReductionToElementWise`
This improves code reuse, enables small simplifications, and unifies the
structure of the patterns. Add high-level comments to clarify the
overall lowering strategy.
Prepares for future refactoring (e.g. #182301) and helps maintain a
uniform implementation.