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

[Nikita Popov via llvm-commits]

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nikic wrote:

Please also add negative tests where intrinsic IDs are not the same or vector types don't match, and add a test that should flags are not preserved.

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