[llvm] [InstCombine][NFC] Precommit a test for folding a binary op of reductions. (PR #121568)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM, though adding one test using scalable vectors probably wouldn't hurt.

https://github.com/llvm/llvm-project/pull/121568