Separates actual transformation files from supporting utility files in
the transforms directory. Includes a bazel overlay fix for the build (as
well as a bit of cleanup of that file to be less verbose and more
flexible).
Rewrite patterns must return `success` if the IR was modified. This
commit fixes sparse tensor tests such as
`SparseTensor/sparse_fusion.mlir`,
`SparseTensor/CPU/sparse_reduce_custom.mlir`,
`SparseTensor/CPU/sparse_semiring_select.mlir` when running with
`MLIR_ENABLE_EXPENSIVE_PATTERN_API_CHECKS`.
Rather than extending sparsifier codegen with higher order
non-permutations, we follow the path of rewriting linalg generic ops
into higher order operations. That way, code generation will simply work
out of the box. This is a very first proof-of-concept rewriting of that
idea.
