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

[Pierre van Houtryve via llvm-commits]

https://github.com/Pierre-vh approved this pull request.

LGTM if nits addressed, after fixing them wait a bit to see if other reviewers have objections too

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