[llvm] [TableGen] Fix concatenation of subreg and artificial subregs (PR #114391)

[Sander de Smalen via llvm-commits]

sdesmalen-arm wrote:

> The code change looks fine to me. I'd still prefer to see minimal tests for the original problem, and for the new cases that [588de85](https://github.com/llvm/llvm-project/commit/588de8521a2cdbc9a67c2c2136bd31dcb8f3c792) will catch, but I won't insist.

Thanks for giving it another look! I've reduced the CHECK lines a bit, which hopefully makes the test a bit easier to parse.

https://github.com/llvm/llvm-project/pull/114391