[LoopUnrolling] Peel loops with invariant backedge Phi input

Summary:
If a loop contains a Phi node which has an invariant input from back
edge, it is profitable to peel such loops (rather than unroll them) to
use the advantage that this Phi is always invariant starting from 2nd
iteration. After the 1st iteration is peeled, other optimizations can
potentially simplify calculations with this invariant.

Patch by Max Kazantsev!

Reviewers: sanjoy, apilipenko, igor-laevsky, anna, mkuper, reames

Reviewed By: mkuper

Subscribers: mkuper, mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D30161

llvm-svn: 296898