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

[Alexis Engelke via llvm-commits]

aengelke wrote:

> Such changes are quite rare and usually need just a few straightforward changes in Sandbox IR.

> As far as I am aware the components have not been a maintenance burden, so I don't see the urgency.

Just to give a small data point here: I recently tried to improve block successor iteration, which requires changes to IR data structures (e.g. [here](https://github.com/llvm/llvm-project/commit/c9d698154b2ae14620c7cc4c0c323331aa6de075)). Obviously, this also needs SandboxIR changes, which does cost time to read into, get familiar with how it works, etc., and as I'm not familiar with SandboxIR at all, nothing is really straightforward for me. So there is a very real cost from having this in-tree and built by default.

IR changes are already very time consuming (and allocating time for upstream LLVM work is already hard for many, certainly for me), so I would appreciate if I at least could say "fixing SandboxIR is someone else's problem" (similar to some back-ends).

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