Switch BBVectorize to directly depend on having a TTI analysis.

This could be simplified further, but Hal has a specific feature for
ignoring TTI, and so I preserved that.

Also, I needed to use it because a number of tests fail when switching
from a null TTI to the NoTTI nonce implementation. That seems suspicious
to me and so may be something that you need to look into Hal. I worked
it by preserving the old behavior for these tests with the flag that
ignores all target info.