Implement basic array prefetching hints in Hydrogen.

src/hydrogen-instructions.cc

Index: src/hydrogen-instructions.cc
===================================================================
--- src/hydrogen-instructions.cc	(revision 13123)
+++ src/hydrogen-instructions.cc	(working copy)
@@ -2021,6 +2021,8 @@
   }
 
   dependency()->PrintNameTo(stream);
+  stream->Add(" ");
+  prefetch_distance()->PrintNameTo(stream);
   if (RequiresHoleCheck()) {
     stream->Add(" check_hole");
   }
@@ -2074,6 +2076,7 @@
             index_cache,
             key_load->key(),
             key_load->key(),
+            key_load->prefetch_distance(),
             key_load->elements_kind());
         map_check->InsertBefore(this);
         index->InsertBefore(this);