LoopVectorizer: Fix a bug in the vectorization of BinaryOperators. The BinaryOperator can be folded to an Undef, and we don't want to set NSW flags to undef vals.

PR14878



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172079 91177308-0d34-0410-b5e6-96231b3b80d8