This adds a rewrite that converts illegal 2D unit-dim `shape_casts` into
`vector.transpose` ops.
E.g.
```mlir
// Case 1:
%a = vector.shape_cast %0 : vector<[4]x1xf32> to vector<1x[4]xf32>
// Case 2:
%b = vector.shape_cast %1 : vector<[4]x1xf32> to vector<[4]xf32>
```
Becomes:
```mlir
// Case 1:
%a = vector.transpose %0 : [1, 0] vector<[4]x1xf32> to vector<1x[4]xf32>
// Case 2:
%t = vector.transpose %1 : [1, 0] vector<[4]x1xf32> to vector<1x[4]xf32>
%b = vector.shape_cast %t : vector<1x[4]xf32> to vector<[4]xf32>
```
Various lowerings and drop unit-dims patterns add such shape_casts,
however, if they do not cancel out (which they likely won't if we've
reached the vector-legalization pass) they will prevent lowering the IR.
Rewriting them as a transpose gives `LiftIllegalVectorTransposeToMemory`
a chance to eliminate the illegal types.
d1fc59c3b5