[mlir][scf] Loop peeling: Use scf.for for partial iteration

Generate an scf.for instead of an scf.if for the partial iteration. This is for consistency reasons: The peeling of linalg.tiled_loop also uses another loop for the partial iteration.

Note: Canonicalizations patterns may rewrite partial iterations to scf.if afterwards.

Differential Revision: https://reviews.llvm.org/D109568