[vectorizer] Add some flags which are useful for conducting experiments

with the unrolling behavior in the loop vectorizer. No functionality
changed at this point.

These are a bit hack-y, but talking with Hal, there doesn't seem to be
a cleaner way to easily experiment with different thresholds here and he
was also interested in them so I wanted to commit them. Suggestions for
improvement are very welcome here.

llvm-svn: 200212