An existing, previously xfailed test that I recently removed
the xfail from made me realize that -Wstringop-overflow doesn't
properly detect buffer overflow resulting from vectorized stores.
Because of a difference in the IL the test passes on x86_64 but
fails on targets like aarch64.  Other examples can be constructed
that -Wstringop-overflow fails to diagnose even on x86_64.  For
INSTANCE, the overflow in the following function isn't diagnosed
when the loop is vectorized:

   void* f (void)
   {
     char *p = __builtin_malloc (8);
     for (int i = 0; i != 16; ++i)
       p[i] = 1 << i;
     return p;
   }

The attached change enhances the warning to detect those as well.
It found a few bugs in vectorizer tests that the patch corrects.
Tested on x86_64-linux and with an aarch64 cross.

Martin