[mlir][Linalg] Cleanup the drop unit dims pass in Linalg.

TL;DR the following API functions have been merged

```
void populateFoldUnitExtentDimsViaReshapesPatterns(RewritePatternSet &patterns);
void populateFoldUnitExtentDimsViaSlicesPatterns(RewritePatternSet &patterns);
```

into

```
void populateFoldUnitExtentDimsPatterns(RewritePatternSet &patterns,
                                        ControlDropUnitDims &options);
```

To use the previous functionality use

```
ControlDropUnitDims options;
// By default options.rankReductionStrategy is
// ControlDropUnitDims::RankReductionStrategy::ReassociativeReshape.
populateFoldUnitExtentDimsPatterns(patterns, options);
```

and

```
ControlDropUnitDims options;
options.rankReductionStrategy = ControlDropUnitDims::RankReductionStrategy::ExtractInsertSlice
populateFoldUnitExtentDimsPatterns(patterns, options);

```

This pass is quite old and needed to be updated based on the current
approach to transformations in Linalg

- Instead of two patterns, one to just remove loop dimensions that are
  unit extent (and using 0 in the indexing maps), and another to drop
  the unit-extents in the operand shapes, combine into a single
  transformation. This avoid creating an intermediate step with
  indexing maps having 0's in the domains exp ressions.

- Expose the core transformation as a utility function and add a
  pattern that calls this transformation.

This is a mostly NFC change, apart from the API change and dropping
the patterns/test that only dropped the loops that are unit extents.

Differential Revision: https://reviews.llvm.org/D155518