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