As a follow-up to PR #135841 (see discussion for background), this patch
removes the `ConvertIllegalShapeCastOpsToTransposes` pattern from the SME
legalization pass. This change unblocks folding for ShapeCastOp involving
scalable vectors.
Originally, the `ConvertIllegalShapeCastOpsToTransposes` pattern was introduced
to rewrite certain `vector.shape_cast` ops that could not be lowered otherwise.
Based on local end-to-end testing, this workaround is no longer required, and
the pattern can now be safely removed.
This patch also removes a special case from `ShapeCastOp::fold`, simplifying
the fold logic.
As a side effect of removing `ConvertIllegalShapeCastOpsToTransposes`, we lose
the mechanism that enabled lowering of certain ops like:
```mlir
%res = vector.transfer_read %mem[%a, %b] (...) :
memref<?x?xf32>, vector<[4]x1xf32>
```
Previously, such cases were handled by:
* Rewriting a nearby `vector.shape_cast` to a `vector.transpose` (via
`ConvertIllegalShapeCastOpsToTransposes`)
* Then lowering the result with `LiftIllegalVectorTransposeToMemory`.
This patch introduces a new dedicated pattern,
`LowerColumnTransferReadToLoops`, that directly handles illegal
`vector.transfer_read` ops involving leading scalable dimensions.
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