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

[Amara Emerson via llvm-commits]

https://github.com/aemerson commented:

This feels like something that should be automatically simplified away by the user of the concat, using the ArtifactValueFinder. We should be able to look through the concat of `{source_val, undef, undef, undef}` and directly forward the `source_val` register to the user.

The idea is that we as much as possible avoid adding fragile combines/transformations to the artifact combiner to keep it stable and resilient to changing IR inputs.

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