VM: Improve phi-elimination to improve code inside of try-catch.

runtime/vm/redundancy_elimination.cc

Index: runtime/vm/redundancy_elimination.cc
diff --git a/runtime/vm/redundancy_elimination.cc b/runtime/vm/redundancy_elimination.cc
index adc131995d27c87fd9ca5f9c5cdd683056dbd1af..b24f5587034aef0d0c247bcd11cb2ac28fba37f8 100644
--- a/runtime/vm/redundancy_elimination.cc
+++ b/runtime/vm/redundancy_elimination.cc
@@ -2347,6 +2347,8 @@ class LoadOptimizer : public ValueObject {
   // Insert the given phi into the graph. Attempt to find an equal one in the
   // target block first.
   // Returns true if the phi was inserted and false if it was replaced.
+  // TODO(fschneider): Generalize this to allow replacing a phi with a non-phi
+  // definition as well so that we don't need Phi::HasReplacement anymore.
   bool EmitPhi(PhiInstr* phi) {
     for (PhiIterator it(phi->block()); !it.Done(); it.Advance()) {
       if (ReplacePhiWith(phi, it.Current())) {
@@ -3437,6 +3439,7 @@ void DeadCodeElimination::EliminateDeadPhis(FlowGraph* flow_graph) {
       for (intptr_t i = 0; i < join->phis_->length(); ++i) {
         PhiInstr* phi = (*join->phis_)[i];
         if (phi != NULL) {
+          Definition* replacement = NULL;
           if (!phi->is_alive()) {
             phi->ReplaceUsesWith(flow_graph->constant_null());
             phi->UnuseAllInputs();
@@ -3450,7 +3453,16 @@ void DeadCodeElimination::EliminateDeadPhis(FlowGraph* flow_graph) {
             (*join->phis_)[i] = NULL;
             if (FLAG_trace_optimization) {
               THR_Print("Removing redundant phi v%" Pd "\n",
-                         phi->ssa_temp_index());
+                        phi->ssa_temp_index());
+            }
+          } else if (phi->HasReplacement(&replacement)) {
+            phi->ReplaceUsesWith(replacement);
+            phi->UnuseAllInputs();
+            (*join->phis_)[i] = NULL;
+            if (FLAG_trace_optimization) {
+              THR_Print("Replace redundant phi v%" Pd " with v%" Pd "\n",
+                        phi->ssa_temp_index(),
+                        replacement->ssa_temp_index());
             }
           } else {
             (*join->phis_)[to_index++] = phi;