[mlir] introduce transform.loop.forall_to_for (#65474)

Add a straightforward sequentialization transform from `scf.forall` to a
nest of `scf.for` in absence of results and expose it as a transform op.
This is helpful in combination with other transform ops, particularly
fusion, that work best on parallel-by-construction `scf.forall` but
later need to target sequential `for` loops.