Add loop printing to block comments.

runtime/vm/flow_graph.h

Index: runtime/vm/flow_graph.h
===================================================================
--- runtime/vm/flow_graph.h	(revision 39664)
+++ runtime/vm/flow_graph.h	(working copy)
@@ -222,13 +222,21 @@
   // after this point.
   void disallow_licm() { licm_allowed_ = false; }
 
-  const ZoneGrowableArray<BlockEntryInstr*>& loop_headers() {
+  const ZoneGrowableArray<BlockEntryInstr*>& LoopHeaders() {
     if (loop_headers_ == NULL) {
       loop_headers_ = ComputeLoops();
     }
     return *loop_headers_;
   }
 
+  const ZoneGrowableArray<BlockEntryInstr*>* loop_headers() const {
+    return loop_headers_;
+  }
+
+  // Finds natural loops in the flow graph and attaches a list of loop
+  // body blocks for each loop header.
+  ZoneGrowableArray<BlockEntryInstr*>* ComputeLoops() const;
+
   // Per loop header invariant loads sets. Each set contains load id for
   // those loads that are not affected by anything in the loop and can be
   // hoisted out. Sets are computed by LoadOptimizer.
@@ -295,12 +303,8 @@
   // "Advanced Compiler Design & Implementation" (Muchnick) p192.
   // Returns a BitVector indexed by block pre-order number where each bit
   // indicates membership in the loop.
-  BitVector* FindLoop(BlockEntryInstr* m, BlockEntryInstr* n);
+  BitVector* FindLoop(BlockEntryInstr* m, BlockEntryInstr* n) const;
 
-  // Finds natural loops in the flow graph and attaches a list of loop
-  // body blocks for each loop header.
-  ZoneGrowableArray<BlockEntryInstr*>* ComputeLoops();
-
   Isolate* isolate_;
 
   // DiscoverBlocks computes parent_ and assigned_vars_ which are then used