[llvm] SLPVectorizer: Use use_empty instead of getNumUses (PR #136336)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Apr 18, 3:12 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/136336).


https://github.com/llvm/llvm-project/pull/136336