[llvm] [TableGen] !subst on a dag should retain name of operator (PR #141195)

[Adam Nemet via llvm-commits]

anemet wrote:

> LGTM.
> 
> Nit: this change has nothing to do with backend-related concepts, it is pure tablegen. I would have made the test more "symbolic":
> 
> ```
> dag d = (set X:$x, (Y:$y X:$a, X:$b));
> dag n = !foreach(i, d, !subst(X, Z, i));
> ```

(set) is still backend concept :).  I think it's good to have a connection to real life (backend) so keeping it.

https://github.com/llvm/llvm-project/pull/141195