Decouple affine->standard lowering from the pass

    The lowering from the Affine dialect to the Standard dialect was originally
    implemented as a standalone pass.  However, it may be used by other passes
    willing to lower away some of the affine constructs as a part of their
    operation.  Decouple the transformation functions from the pass infrastructure
    and expose the entry point for the lowering.

    Also update the lowering functions to use `LogicalResult` instead of bool for
    return values.

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PiperOrigin-RevId: 250229198