Move logic to enable the vectorizer to clang

We used to decide whether to really vectorize depending on the optimization
level in PassManagerBuilder.

This patch moves this decision to the clang driver. We look at the optimization
level and whether the f(no-)vectorize is set and decide whether to vectorize.

This allows us to simplify the logic in PassManagerBuilder to just a check for
whether the vectorizer should run or not.

We now do the right thing for:
$ clang -O1 -fvectorize
$ clang -fno-vectorize -O3