LoopUnroll: Only check for ScalarEvolution analysis once, NFC
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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@219239 91177308-0d34-0410-b5e6-96231b3b80d8

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