Add a simple dominator based redundancy elimination.

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
===================================================================
--- runtime/vm/compiler.cc	(revision 11221)
+++ runtime/vm/compiler.cc	(working copy)
@@ -34,7 +34,7 @@
 DEFINE_FLAG(bool, disassemble_optimized, false, "Disassemble optimized code.");
 DEFINE_FLAG(bool, trace_bailout, false, "Print bailout from ssa compiler.");
 DEFINE_FLAG(bool, trace_compiler, false, "Trace compiler operations.");
-DEFINE_FLAG(bool, local_cse, true, "Do local subexpression elimination.");
+DEFINE_FLAG(bool, cse, true, "Do common subexpression elimination.");
 DEFINE_FLAG(int, deoptimization_counter_threshold, 5,
     "How many times we allow deoptimization before we disallow"
     " certain optimizations");
@@ -189,9 +189,8 @@
         // Do optimizations that depend on the propagated type information.
         optimizer.OptimizeComputations();
 
-        if (FLAG_local_cse) {
-          LocalCSE local_cse(*flow_graph);
-          local_cse.Optimize();
+        if (FLAG_cse) {
+          DominatorBasedCSE::Optimize(flow_graph->graph_entry());
         }
 
         // Perform register allocation on the SSA graph.