[mlir][SCF] convert-scf-to-cf: Lower scf.forall to scf.parallel (#65449)

scf.forall ops without shared outputs (i.e., fully bufferized ops) are
lowered to scf.parallel. scf.forall ops are typically lowered by an
earlier pass depending on the execution target. E.g., there are
optimized lowerings for GPU execution. This new lowering is for
completeness (convert-scf-to-cf can now lower all SCF loop constructs)
and provides a simple CPU lowering strategy for testing purposes.

scf.parallel is currently lowered to scf.for, which executes
sequentially. The scf.parallel lowering could be improved in the future
to run on multiple threads.