[LoopUnroll] Only peel if a predicate becomes known in the loop body.

If a predicate does not become known after peeling, peeling is unlikely
to be beneficial.

Reviewers: mcrosier, efriedma, mkazantsev, junbuml

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D44983

llvm-svn: 330250