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

[Mel Chen via llvm-commits]

Mel-Chen 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.

I tried, it is worked well. 
@artagnon I guess the target you tried doesn't support popcount instruction. Please enable it and try again.

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