LoopUnroll: Only check for ScalarEvolution analysis once, NFC

A follow-up commit will add use to a tight loop.  We might as well just
find it once anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219239 91177308-0d34-0410-b5e6-96231b3b80d8