[MLIR] [Transforms] Let `transform.structured.convert_to_loops` return handles to loops (#83984)

This lets `transform.structured.convert_to_loops` return handles to the
generated loops, making this transformation more useful to use for
(transformation-)nesting purposes. This is modelled after SCFs
`transform.loop.forall_to_for` which returns handles to loops.

Introduced in commit aa2a96a2, with a
note that they might move out of the `Linalg`-Dialect, but no reason
given for the non-return of handles. As far as I can see, this transform
always returns loops.