ScheduleOptimizer: Fix prevectorization.

In case we are at the innermost band, we try to prepare for vectorization. This
means, we look for the innermost parallel loop and strip mine this loop to the
innermost level using a strip-mine factor corresponding to the number of vector
iterations.

For whatever reason, the code that implemented this feature was broken. We now
added a comment, a test case and obviously also the right code.