[LoopVectorize] Don't preserve nsw/nuw flags on shrunken ops.

If we're shrinking a binary operation, it may be the case that the new
operations wraps where the old didn't. If this happens, the behavior
should be well-defined. So, we can't always carry wrapping flags with us
when we shrink operations.

If we do, we get incorrect optimizations in cases like:

void foo(const unsigned char *from, unsigned char *to, int n) {
  for (int i = 0; i < n; i++)
    to[i] = from[i] - 128;
}

which gets optimized to:

void foo(const unsigned char *from, unsigned char *to, int n) {
  for (int i = 0; i < n; i++)
    to[i] = from[i] | 128;
}

Because:
- InstCombine turned `sub i32 %from.i, 128` into
  `add nuw nsw i32 %from.i, 128`.
- LoopVectorize vectorized the add to be `add nuw nsw <16 x i8>` with a
  vector full of `i8 128`s
- InstCombine took advantage of the fact that the newly-shrunken add
  "couldn't wrap", and changed the `add` to an `or`.

InstCombine seems happy to figure out whether we can add nuw/nsw on its
own, so I just decided to drop the flags. There are already a number of
places in LoopVectorize where we rely on InstCombine to clean up.

llvm-svn: 305053