Use the instruction iterator rather than an explicit loop in more places.

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
diff --git a/runtime/vm/flow_graph_builder.cc b/runtime/vm/flow_graph_builder.cc
index 833b12a2c868a3d60782241d7aa02c071172d454..d668e63c1a3dd8134a990bb4d14d4acc4f06727d 100644
--- a/runtime/vm/flow_graph_builder.cc
+++ b/runtime/vm/flow_graph_builder.cc
@@ -2220,12 +2220,12 @@ void FlowGraphBuilder::BuildGraph(bool for_optimized, bool use_ssa) {
   if (for_optimized && use_ssa) {
     // Link instructions backwards for optimized compilation.
     for (intptr_t i = 0; i < block_count; ++i) {
-      Instruction* prev = postorder_block_entries_[i];
-      Instruction* current = prev->next();
-      while (current != NULL && !current->IsBlockEntry()) {
-        current->set_previous(prev);
-        prev = current;
-        current = current->next();
+      BlockEntryInstr* entry = postorder_block_entries_[i];
+      Instruction* previous = entry;
+      for (ForwardInstructionIterator it(entry); !it.Done(); it.Advance()) {
+        Instruction* current = it.Current();
+        current->set_previous(previous);
+        previous = current;
       }
     }
     GrowableArray<BitVector*> dominance_frontier;