[llvm] SPIRV: Use use_empty instead of getNumUses == 0 (PR #136338)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Apr 19, 3:25 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/136338).


https://github.com/llvm/llvm-project/pull/136338