VSETCC must define all bits, which is different than it was documented
to before. Since all targets that implement VSETCC already have this
behavior, and we don't optimize based on this, just change the
documentation. We now get nice code for vec_compare.ll
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74978 91177308-0d34-0410-b5e6-96231b3b80d8