Remove more null receiver guards.

pkg/compiler/lib/src/ssa/optimize.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of ssa;
 
 abstract class OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
@@ skipping 994 matching lines @@
     HInstruction receiver = instruction.getDartReceiver(compiler);
     HInstruction current = instruction.next;
     do {
       if ((current.getDartReceiver(compiler) == receiver)
           && current.canThrow()) {
         return true;
       }
       if (current.canThrow() || current.sideEffects.hasSideEffects()) {
         return false;
       }
-      if (current.next == null && current is HGoto) {
-        // We do not merge blocks in our SSA graph, so if this block
-        // just jumps to a single predecessor, visit this predecessor.
-        assert(current.block.successors.length == 1);
-        current = current.block.successors[0].first;
-      } else {
-        current = current.next;
+      HInstruction next = current.next;
+      if (next == null) {
+        // We do not merge blocks in our SSA graph, so if this block just jumps
+        // to a single successor, visit the successor, avoiding back-edges.
+        HBasicBlock successor;
+        if (current is HGoto) {
+          successor = current.block.successors.single;
+        } else if (current is HIf) {
+          // We also leave HIf nodes in place when one branch is dead.
+          HInstruction condition = current.inputs.first;
+          if (condition is HConstant) {
+            bool isTrue = condition.constant.isTrue;
+            successor = isTrue ? current.thenBlock : current.elseBlock;
+            assert(!analyzer.isDeadBlock(successor));
+          }
+        }
+        if (successor != null && successor.id > current.block.id) {
+          next = successor.first;
+        }
       }
+      current = next;
     } while (current != null);
     return false;
   }
 
   bool isTrivialDeadStoreReceiver(HInstruction instruction) {
     // For an allocation, if all the loads are dead (awaiting removal after
     // SsaLoadElimination) and the only other uses are stores, then the
     // allocation does not escape which makes all the stores dead too.
     bool isDeadUse(HInstruction use) {
       if (use is HFieldSet) {
@@ skipping 1156 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }