LLVM's Loop Vectorizer is used at various compiler optimization levels but its newer SLP Vectorizer hasn't been turned on yet unless explicitly telling it to via a compiler flag (-fslp-vectorize). This may change though with an Apple LLVM developer now proposing the SLP Vectorizer go on for the -O3 performant level.
Back in May I ran benchmarks on the SLP Vectorizer with its introduction in LLVM 3.3. The SLP Vectorizer is about vectorizing straight-line code over loops.
The discussion about enabling the SLP Vectorizer can be found on the mailing list. It hasn't been decided for sure whether the SLP Vectorizer will be turned on, due to possible regressions in select workloads. However, ultimately I would anticipate the SLP Vectorizer to be turned on in due time for LLVM for delivering better performance. If it's turned on soon, it will be a feature of LLVM 3.4 that's due for release before year's end.