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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

Thanks a lot @nikic and @Mel-Chen for the guidance! Indeed, turning on zbb enables LLVM to use ctpop in the example.

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