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

[Francesco Petrogalli via llvm-commits]

https://github.com/fpetrogalli approved this pull request.

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));
```


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