[LoopUnrolling] Fix loop size check for peeling

Summary:
We should check if loop size allows us to peel at least one iteration
before we do so.

Patch by Max Kazantsev!

Reviewers: sanjoy, mkuper, efriedma

Reviewed By: mkuper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30632

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297122 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed