[LoopVectorizer] Limit unroll factor in the presence of nested reductions.

[LoopVectorizer] Limit unroll factor in the presence of nested reductions.

If we have a scalar reduction, we can increase the critical path length if the loop we're unrolling is inside another loop. Limit, by default to 2, so the critical path only gets increased by one reduction operation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216140 91177308-0d34-0410-b5e6-96231b3b80d8