[llvm] DAG: Fix vector bin op scalarize defining a partially undef vector (PR #122459)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jan 16, 8:30 PM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/122459).


https://github.com/llvm/llvm-project/pull/122459