Mention memdep in the AA documentation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42519 91177308-0d34-0410-b5e6-96231b3b80d8

docs/AliasAnalysis.html

@@ -50,6 +50,7 @@
     implementations</a></li>
     </ul>
   </li>
+  <li><a hread="#memdep">Memory Dependence Analysis</a></li>
 </ol>
 
 <div class="doc_author">
@@ -947,6 +948,24 @@ algorithm will have a lower number of may aliases).</p>
 
 </div>
 
+<!-- *********************************************************************** -->
+<div class="doc_section">
+  <a name="memdep">Memory Dependence Analysis</a>
+</div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>If you're just looking to be a client of alias analysis information, consider
+using the Memory Dependence Analysis interface instead.  MemDep is a lazy, 
+caching layer on top of alias analysis that is able to answer the question of
+what preceding memory operations a given instruction depends on, either at an
+intra- or inter-block level.  Because of its laziness and caching 
+policy, using MemDep can be a significant performance win over accessing alias
+analysis directly.</p>
+
+</div>
+
 <!-- *********************************************************************** -->
 
 <hr>