This code in this branch assumes the `src` operand in
`insert_slice_async` always aliases the result, which shouldn't hold for
generally cases but is just a workaround to make the pipeline pass work.
I'm also working on the complete analysis in another
[branch](https://github.com/openai/triton-mlir/tree/keren/analyze-slice).
This PR both simplifies the layout conversion simplification algorithm, and also improves it to make it work with vectorized element-wise ops. The conversion optimizer still has a lot of room for improvements, and other PRs will address its limitations (ideally via some sort of explicit cost model)
