Don't emit nsw flags for vector operations; there's basically no benefit, and a lot...

Don't emit nsw flags for vector operations; there's basically no benefit, and a lot of downside (like PR9850, which is about clang's xmmintrin.h making an unexpected transformation on an expression involving _mm_add_epi32).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@131000 91177308-0d34-0410-b5e6-96231b3b80d8