[llvm] LoopVectorize/test: clean up reduction.ll; generate using UTC (NFC) (PR #68890)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> If you're using UTC, there should not be any CHECK-NOT lines.

I'm conforming to the practice used in the (recent) iv-select-cmp.ll. I regenerate using UTC, and stage all hunks except the CHECK-NOT checks. I think it's clearer this way: we convey the information that the example is not vectorized, without being overly verbose.

https://github.com/llvm/llvm-project/pull/68890