[MLIR][NFC] Rename MemRefDataFlow -> AffineScalarReplacement

NFC. Rename MemRefDataFlow -> AffineScalarReplacement and move to
AffineTransforms library. Pass command line rename: -memref-dataflow-opt
-> affine-scalrep. Update outdated pass documentation.

Rationale:
https://llvm.discourse.group/t/move-and-rename-memref-dataflow-opt-lib-transforms-lib-affine-dialect-transforms/3640

Differential Revision: https://reviews.llvm.org/D104190