`combineContractAndBroadcast` is a pattern that matches
`vector.contract` operations whose inputs are defined by
`vector.broadcast`, e.g.:
```mlir
%0 = vector.broadcast %lhs : vector<8x4xi32> to vector<1x8x4xi32>
%1 = vector.broadcast %rhs : vector<8x4xi32> to vector<1x8x4xi32>
%result = vector.contract {
indexing_maps = [#map0, #map1, #map2],
iterator_types = ["reduction", "parallel", "parallel", "reduction"],
kind = #vector.kind<add>
} %0, %1, %acc : vector<1x8x4xi32>, vector<1x8x4xi32> into vector<8x8xi32>
```
The pattern folds away the reduction over a unit dimension introduced by
the defining operations, producing a simplified `vector.contract`:
```mlir
vector.contract {
indexing_maps = [#[[$MAP0]], #[[$MAP1]], #[[$MAP2]]]
iterator_types = ["parallel", "parallel", "reduction"]
} %0, %1, %acc : vector<8x4xi32>, vector<8x4xi32> into vector<8x8xi32>
```
This change extends the pattern to also accept broadcast-like
`vector.shape_cast` operations:
```mlir
%0 = vector.shape_cast %lhs : vector<8x4xi32> to vector<1x8x4xi32>
%1 = vector.shape_cast %rhs : vector<8x4xi32> to vector<1x8x4xi32>
%result = vector.contract {
indexing_maps = [#map0, #map1, #map2],
iterator_types = ["reduction", "parallel", "parallel", "reduction"],
kind = #vector.kind<add>
} %0, %1, %acc : vector<1x8x4xi32>, vector<1x8x4xi32> into vector<8x8xi32>
```
Note that `mlir::vector::isBroadcastableTo` was only moved to make it
available to the newly introduced `ShapeCastOp` helper,
`ShapeCastOp::isBroadcastLike()`.
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