Update the vectorizer docs.

    <b>-mllvm -force-vector-width=4</b>.

    The default value is <b>0</b> which means auto-select.

    <br/>

    We can now vectorize this code:

    We can now vectorize this function:

    <pre class="doc_code">

    for (i=0; i<n; i++) {

      a[i] = b[i+1] + c[i+3] + i;

      sum += d[i];

    unsigned sum_arrays(int *A, int *B, int start, int end) {

      unsigned sum = 0;

      for (int i = start; i < end; ++i)

        sum += A[i] + B[i] + i;

      return sum;

    }

    </pre>

    We vectorize under the following loops:

    <ul>

    <li>The inner most loops must have a single basic block.</li>

    <li>The number of iterations are known before the loop starts to execute.</li>

    <li>The loop counter needs to be incrimented by one.</li>

    <li>The loop trip count <b>can</b> be a variable.</li>

    <li>Loops do <b>not</b> need to start at zero.</li>

    <li>The induction variable can be used inside the loop.</li>

    <li>Loop reductions are supported.</li>

    <li>Arrays with affine access pattern do <b>not</b> need to be marked as 'noalias' and are checked at runtime.</li>

    <li>...</li>

    </ul>

</p>

<p>SROA - We've re-written SROA to be significantly more powerful.