[llvm] LoopVectorize/reduction: add negative test for vectorizable example (PR #68912)

[Nikita Popov via llvm-commits]

nikic wrote:

LIR actually already has support for recognizing popcounts: https://github.com/llvm/llvm-project/blob/50ece4cba949787241b5fbfc94be6cfdc66e90ee/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp#L1861

So you'd want to find out why that doesn't trigger and maybe add a PhaseOrdering test.

https://github.com/llvm/llvm-project/pull/68912