[llvm] [SandboxIR] Delete SandboxIR/SandboxVectorizer (PR #166417)

[via llvm-commits]

vporpo wrote:

>  It's still only used by the Sandbox Vectorizer though, and if we're looking at moving that out of tree, I think removing Sandbox IR at the same time also makes sense.

The decision why this project was developed in tree in the first place was discussed in the [RFC](https://discourse.llvm.org/t/rfc-sandbox-vectorizer-an-experimental-modular-vectorizer/79059). I am not sure why this is being brought up again.

>   it does not seem like there are any plans/interest in significantly changing this

How did you come to this conclusion? Just because development is slow, it doesn't mean that there are no plans or interest in the project. 
Please bear in mind that the project has been unfunded for one year and it was during this period that it became stable and has seen more than 100 patches land.

> Yes, I saw that patch. But there haven't been any patches that have landed since June, so it seems like development has stalled.

There are plenty of components in LLVM without patches in the last couple of months. In the Sandbox Vectorizer's case there are at least pending patches in the pipeline. The only reason they haven't been checked in yet, is because I am trying to stick to the LLVM community's code review standards and get them thoroughly reviewed upstream by developers who are familiar with the codebase before landing. Just because a reviewer might be temporarily unavailable, it doesn't suggest that they won't be available in the future.

Regarding the maintenance cost, I would argue that it hasn't been a burden to anyone yet. Nobody so far has raised any actual concerns regarding code maintenance, at least nobody has reached out to me. As the active maintainer, I am always available to help with any issues that show up.

I feel it's a bit too early to suggest moving the project out of tree. As far as I am aware the components have not been a maintenance burden, so I don't see the urgency. The Sandbox Vectorizer already provides a really nice toolset of modular components that can be used to build modular vectorization pipelines.

https://github.com/llvm/llvm-project/pull/166417