[mlir-reduce] Reducer refactor.

* A Reducer is a kind of RewritePattern, so it's just the same as
writing graph rewrite.
* ReductionTreePass operates on Operation rather than ModuleOp, so that
* we are able to reduce a nested structure(e.g., module in module) by
* self-nesting.

Reviewed By: jpienaar, rriddle

Differential Revision: https://reviews.llvm.org/D101046