[mlir][bufferize] Fix config not passed to greedy rewriter

Also add a TODO to switch to a custom walk instead of the GreedyPatternRewriter, which should be more efficient. (The bufferization pattern is guaranteed to apply only a single time for every op, so a simple walk should suffice.)

We currently specify a top-to-bottom walk order. This is important because other walk orders could introduce additional casts and/or buffer copies. These canonicalize away again, but it is more efficient to never generate them in the first place.

Note: A few of these canonicalizations are not yet implemented.

Differential Revision: https://reviews.llvm.org/D121518