[llvm] llvm-reduce: Defer a shouldKeep call in operand reduction (PR #133387)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Apr 2, 3:59 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/133387).


https://github.com/llvm/llvm-project/pull/133387