Subviews are supposed to be expanded before we hit the lowering code.
The expansion is done with the pass called expand-strided-metadata.
Add a test that demonstrate how these passes can be linked up to achieve the desired lowering.
This patch is NFC in spirit but not in practice because subview gets lowered into reinterpret_cast(extract_strided_metadata, <some math>) which lowers in two memref descriptors (one for reinterpert_cast and one for extract_strided_metadata), which creates some noise of the form: extractvalue(unrealized_cast(extractvalue[0]))[0] that is currently not simplified within MLIR but that is really just noop in that case.
I'd expect -reconcile-unrealized-casts to fix that.