[mlir][bufferize][NFC] Use custom walk instead of GreedyPatternRewriter

The bufferization driver was previously using a GreedyPatternRewriter. This was problematic because bufferization must traverse ops top-to-bottom. The GreedyPatternRewriter was previously configured via `useTopDownTraversal`, but this was a hack; this API was just meant for performance improvements and should not affect the result of the rewrite.

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Differential Revision: https://reviews.llvm.org/D123618