[llvm] [AArch64][GlobalISel] Combine G_CONCAT_VECTORS with Illegal Form (PR #85047)

[Amara Emerson via llvm-commits]

https://github.com/aemerson commented:

I think this is a very specific transform to get around our `<4 x s8>` issue but unfortunately I don't have a better suggestion. 

https://github.com/llvm/llvm-project/pull/85047