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

[Jay Foad via llvm-commits]

jayfoad 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 588de8521a2cdbc9a67c2c2136bd31dcb8f3c792 will catch, but I won't insist.

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