[mlir][linalg] Add option to generate rank-reducing slices in DropUnitDims

This change extends the `ReplaceUnitExtents` pattern so that users can choose between of two strategies for generating rank reductions:
* CollapseShapeOp / ExpandShapeOp (was already implemented but code was cleaned up; default strategy)
* rank-reducing ExtractSliceOp / InsertSliceOp

Also add helper functions to the memref dialect that we already have on the tensor dialect: `getMixedSizes`, `createCanonicalRankReducingSubViewOp`, `rankReduceIfNeeded`.

We are using ReassociationIndices instead of ReassoicationExprs in many other places and this makes the code easier to read. Also adding a new test case (that also passed before).

Differential Revision: https://reviews.llvm.org/D139947