LoopVectorizer: A reduction that has multiple uses of the reduction value is not
a reduction.

Really. Under certain circumstances (the use list of an instruction has to be
set up right - hence the extra pass in the test case) we would not recognize
when a value in a potential reduction cycle was used multiple times by the
reduction cycle.

Fixes PR18526.
radar://15851149

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199570 91177308-0d34-0410-b5e6-96231b3b80d8