[RFC] Allow loop vectorizer to choose vector widths that generate illegal types

Thanks, Ayal!

Experimenting with increasing the VF search space beyond the size of the machine vector for loops with mixed types makes sense to me.

In a separate thread you mentioned that the cost model is inaccurate for some cross-vector trunc/exts. It should be easy to improve the quality of the estimation for the common operations. It should just be a matter of plugging a new value in the cost table that overrides the legalization logic in the cost model.