[LV] Fold-tail flag

This is the compiler-flag equivalent of the Predicate pragma
(https://reviews.llvm.org/D65197), to direct the vectorizer to fold the
remainder-loop into the main-loop using predication.

Differential Revision: https://reviews.llvm.org/D66108

Reviewers: Ayal, hsaito, fhahn, SjoerdMeije
llvm-svn: 368801