[mlir][ArmSME] Add rewrite to lift illegal vector.transposes to memory (#80170)
When unrolling the reduction dimension of something like a matmul for
SME, you can end up with transposed reads of illegal types, like so:
```mlir
%illegalRead = vector.transfer_read %memref[%a, %b]
: memref<?x?xf32>, vector<[8]x4xf32>
%legalType = vector.transpose %illegalRead, [1, 0]
: vector<[8]x4xf32> to vector<4x[8]xf32>
```
Here the `vector<[8]x4xf32>` is an illegal type, there's no way to lower
a scalable vector of fixed vectors. However, as the final type
`vector<4x[8]xf32>` is legal, we can instead lift the transpose to
memory (producing a strided memref), and eliminate all the illegal
types. This is shown below.
```mlir
%readSubview = memref.subview %memref[%a, %b] [%c8_vscale, %c4] [%c1, %c1]
: memref<?x?xf32> to memref<?x?xf32>
%transpose = memref.transpose %readSubview (d0, d1) -> (d1, d0)
: memref<?x?xf32> to memref<?x?xf32>
%legalType = vector.transfer_read %transpose[%c0, %c0]
: memref<?x?xf32>, vector<4x[8]xf32>
```
parent
42b5b720
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