LoopVectorize: Change API call to get the backedge taken count

Use ScalarEvolution's getBackedgeTakenCount API instead of getExitCount since
that is really what we want to know. Using the more specific getExitCount was
safe because we made sure that there is only one exiting block.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183047 91177308-0d34-0410-b5e6-96231b3b80d8