[LV] Allow reductions that have several uses outside the loop

[LV] Allow reductions that have several uses outside the loop

We currently check whether a reduction has a single outside user. We don't
really need to require that - we just need to make sure a single value is
used externally. The number of external users of that value shouldn't actually
matter.

Differential Revision: https://reviews.llvm.org/D28830

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292424 91177308-0d34-0410-b5e6-96231b3b80d8