LoopVectorize: External uses must use the last value in a reduction cycle

Otherwise, we don't perform operations that would have been performed on
the scalar version.

Fixes PR17498.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192133 91177308-0d34-0410-b5e6-96231b3b80d8