This implements TTI hook 'preferPredicateOverEpilogue' for MVE.  This is a
first version and it operates on single block loops only. With this change, the
vectoriser will now determine if tail-folding scalar remainder loops is
possible/desired, which is the first step to generate MVE tail-predicated
vector loops.

This is disabled by default for now. I.e,, this is depends on option
-disable-mve-tail-predication, which is off by default.

I will follow up on this soon with a patch for the vectoriser to respect loop
hint 'vectorize.predicate.enable'. I.e., with this loop hint set to Disabled,
we don't want to tail-fold and we shouldn't query this TTI hook, which is
done in D70125.

Differential Revision: https://reviews.llvm.org/D69845