[LV] Fallback strategies if tail-folding fails

This implements 2 different vectorisation fallback strategies if tail-folding
fails: 1) don't vectorise at all, or 2) vectorise using a scalar epilogue. This
can be controlled with option -prefer-predicate-over-epilogue, that has been
changed to take a numeric value corresponding to the tail-folding preference
and preferred fallback.

Patch by: Pierre van Houtryve, Sjoerd Meijer.

Differential Revision: https://reviews.llvm.org/D79783