[LV] Tail-Loop Folding

This allows folding of the scalar epilogue loop (the tail) into the main
vectorised loop body when the loop is annotated with a "vector predicate"
metadata hint. To fold the tail, instructions need to be predicated (masked),
enabling/disabling lanes for the remainder iterations.

Differential Revision: https://reviews.llvm.org/D65197

llvm-svn: 367592