[LoopVectorize] Keep hints from original loop on the vector loop

We need to keep loop hints from the original loop on the new vector loop.
Failure to do this meant that, for example:

  void foo(int *b) {
  #pragma clang loop unroll(disable)
    for (int i = 0; i < 16; ++i)
      b[i] = 1;
  }

this loop would be unrolled. Why? Because we'd vectorize it, thus dropping the
hints that unrolling should be disabled, and then we'd unroll it.

llvm-svn: 267970