[llvm] [LoopVectorize][NFC] Disable output for tests that don't need it (PR #124747)

[David Sherwood via llvm-commits]

david-arm wrote:

> LGTM, thanks.
> 
> Presumably this may slightly speed up running some of the tests?

Yeah, I saw a slight improvement, but I suspect the vplan and IR verify code still gets executed. Probably if we disabled the verifiers it would be faster, but we'd have to be careful about doing that.

https://github.com/llvm/llvm-project/pull/124747