[mlir][sparse] force a properly sized view on pos/crd/val under codegen (#91288)

Codegen "vectors" for pos/crd/val use the capacity as memref size, not
the actual used size. Although the sparsifier itself always uses just
the defined pos/crd/val parts, printing these and passing them back to a
runtime environment could benefit from wrapping the basic pos/crd/val
getters into a proper memref view that sets the right size.