Don't create new nodes if we don't need to.

lib/compiler/implementation/ssa/optimize.dart

Index: lib/compiler/implementation/ssa/optimize.dart
diff --git a/lib/compiler/implementation/ssa/optimize.dart b/lib/compiler/implementation/ssa/optimize.dart
index 3245032367aa2fd1a21e06778c3b72451ab91be8..24eb3ae1c46c1ba34c4c43e3f44aa0663adf09ee 100644
--- a/lib/compiler/implementation/ssa/optimize.dart
+++ b/lib/compiler/implementation/ssa/optimize.dart
@@ -679,10 +679,7 @@ class SsaCheckInserter extends HBaseVisitor implements OptimizationPhase {
       index = insertIntegerCheck(node, index);
     }
     index = insertBoundsCheck(node, node.receiver, index);
-    HIndex newInstruction = new HIndex(node.target, node.receiver, index);
-    node.block.addBefore(node, newInstruction);
-    node.block.rewrite(node, newInstruction);
-    node.block.remove(node);
+    node.changeUse(node.index, index);
   }
 
   void visitIndexAssign(HIndexAssign node) {
@@ -693,11 +690,7 @@ class SsaCheckInserter extends HBaseVisitor implements OptimizationPhase {
       index = insertIntegerCheck(node, index);
     }
     index = insertBoundsCheck(node, node.receiver, index);
-    HIndexAssign newInstruction =
-        new HIndexAssign(node.target, node.receiver, index, node.value);
-    node.block.addBefore(node, newInstruction);
-    node.block.rewrite(node, newInstruction);
-    node.block.remove(node);
+    node.changeUse(node.index, index);
   }
 }