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

[Sander de Smalen via llvm-commits]

sdesmalen-arm wrote:

> Personally I'd prefer to see a minimal testcase that shows the effect of the change. (But maybe the 400 line test case is useful too, to catch any intentional or unintentional changes in future?)

I initially started tried to prune the CHECK lines, but then figured it would be useful to catch (un)intentional changes in the future (such as needed for #114392), so opted to keep all lines in the end.

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