See https://github.com/llvm/llvm-project/pull/147168 for more info.

This is a proof of concept recognition of the most basic forms of ReLu
operations, used to show-case sparsification of end-to-end PyTorch
models. In the long run, we must avoid lowering such constructs too
early (with this need for raising them back).

See discussion at

https://discourse.llvm.org/t/min-max-abs-relu-recognition-starter-project/78918

Reverts llvm/llvm-project#81799 ; this broke the mlir gcc7 bot.

…ormat and properties instead.

1. Add parsing methods for block[n, m].
2. Encode n and m with the newly extended 64-bit LevelType enum.
3. Update 2:4 methods names/comments to n:m.

The "Dim" prefix is a legacy left-over that no longer makes sense, since
we have a very strict "Dimension" vs. "Level" definition for sparse
tensor types and their storage.