[llvm] [GlobalISel] Make scalar G_SHUFFLE_VECTOR illegal. (PR #140508)

[Amara Emerson via llvm-commits]

https://github.com/aemerson commented:

I don't have a problem with this in principle, but let's check with @qcolombet in case there's something we're missing.

https://github.com/llvm/llvm-project/pull/140508