[llvm] [SandboxVectorizer] Define SeedBundle: a set of instructions to be vectorized [retry] (PR #111073)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

`<span>` is a C++20 thing and LLVM is on C++20, so it seems like we should be able to use it? although it does look like this is the first use of it. maybe best to not use it for now if so many buildbots are breaking due to this

https://github.com/llvm/llvm-project/pull/111073