There is currently no path to lower scf.forall to scf.parallel with the
goal of targeting the OpenMP dialect.
In the SCF->ControlFlow conversion, scf.forall is briefly converted to
scf.parallel, but the scf.parallel is lowered directly to a sequential
loop. This makes experimenting with scf.forall for CPU execution
difficult.
This change factors out the rewrite in the SCF->ControlFlow pass into a
utility function that can then be used in the SCF->ControlFlow lowering
and via a separate -scf-forall-to-parallel pass.
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Co-authored-by: Spenser Bauman <sabauma@fastmail>
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