[LV] Disable Scalable VFs when tail folding is enabled b/c of low tripcount.

The loop vectorizer may decide to use tail folding when the trip-count
is low. When that happens, scalable VFs are no longer a candidate,
since tail folding/predication is not yet supported for scalable vectors.

This can be re-enabled in a future patch.

Reviewed By: kmclaughlin

Differential Revision: https://reviews.llvm.org/D106657