[llvm] [nfc][ir2vec] Remove `Valid` field (PR #157132)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

### Merge activity

* **Sep 6, 1:07 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/157132).


https://github.com/llvm/llvm-project/pull/157132