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

[via llvm-commits]

https://github.com/vporpo commented:

Regarding the typedef, the user of `getSlice()` expects some kind of array/vector/iterable object. The type itself shouldn't matter, but even if it does we should provide a typedef for 
`MutableArrayRef<SeedList>`, not the nested `SeedList`.

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