[llvm] DAG: Fix vector_shuffle -> splat fold defining undef lanes (PR #123596)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jan 21, 11:54 AM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/123596).


https://github.com/llvm/llvm-project/pull/123596