Add TODO item about generalizing target-specific passes to OpenProjects

[hfinkel]

Summary

We have a number of target-specific passes that probably should be target independent. I think we should add a TODO ltem about these (to investigate generalizing them). In addition to general comments on this idea, please do suggest others that should be added.

OpenProjects.html

 <!-- ======================================================================= -->
 <div class="www_subsubsection">
   <a name="codegen">Code Generator Improvements</a>
 </div>
 
 <div class="www_text">
 
 <ol>
+<li>Generalize target-specific backend passes that could be target-independent,
+    by adding necessary target hooks and making sure all IR/MI features (such as
+    register masks and predicated instructions) are properly handled. Enable these
+    for other targets where doing so is demonstrably beneficial.
+    For example:
+      <ol><li>lib/Target/Hexagon/RDF*</li>
+          <li>lib/Target/AArch64/AArch64AddressTypePromotion.cpp</li>
+     </ol>
+    </li>
 <li>Merge the delay slot filling logic that is duplicated into (at least)
     the Sparc and Mips backends into a single target independent pass.
      Likewise, the branch shortening logic in several targets should be merged
      together into one pass.</li>
 <li>Implement 'stack slot coloring' to allocate two frame indexes to the same
     stack offset if their live ranges don't overlap.  This can reuse a bunch of
     analysis machinery from LiveIntervals.  Making the stack smaller is good
     for cache use and very important on targets where loads have limited