Compute the def-use list on-demand by walking the dominator tree.

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
diff --git a/runtime/vm/compiler.cc b/runtime/vm/compiler.cc
index ed72ea0cac11468664297c04ba2c2529bfe72bd8..e4c5a814f4f3b4a1f27d82c1ed8ec8e9e90453bf 100644
--- a/runtime/vm/compiler.cc
+++ b/runtime/vm/compiler.cc
@@ -163,9 +163,7 @@ static bool CompileParsedFunctionHelper(const ParsedFunction& parsed_function,
       flow_graph = builder.BuildGraph();
 
       // Transform to SSA.
-      if (optimized) {
-        flow_graph->ComputeSSA();
-      }
+      if (optimized) flow_graph->ComputeSSA();
 
       if (FLAG_print_flow_graph) {
         OS::Print("Before Optimizations\n");
@@ -182,10 +180,17 @@ static bool CompileParsedFunctionHelper(const ParsedFunction& parsed_function,
         FlowGraphOptimizer optimizer(*flow_graph);
         optimizer.ApplyICData();
 
+        // Compute the use lists.
+        flow_graph->ComputeUseLists();
+
         // Propagate types and eliminate more type tests.
         FlowGraphTypePropagator propagator(*flow_graph);
         propagator.PropagateTypes();
 
+        // TODO(zerny): Here we assume that the use lists remain valid after
+        // type propagation. We should construct a use-list validator to make
+        // this explicit in DEBUG mode.
+
         // Do optimizations that depend on the propagated type information.
         optimizer.OptimizeComputations();