Make special RPO computation iterative during scheduling.

src/compiler/schedule.h

Index: src/compiler/schedule.h
diff --git a/src/compiler/schedule.h b/src/compiler/schedule.h
index 797418298788cb6c12eebf4e4d3170b2d2e3031d..b65a50719b1bdb2a03956d7e8fb4dee99cc88b46 100644
--- a/src/compiler/schedule.h
+++ b/src/compiler/schedule.h
@@ -145,18 +145,21 @@ class BasicBlock FINAL : public ZoneObject {
   bool deferred() const { return deferred_; }
   void set_deferred(bool deferred) { deferred_ = deferred; }
 
+  int32_t dominator_depth() const { return dominator_depth_; }
+  void set_dominator_depth(int32_t dominator_depth);
+
   BasicBlock* dominator() const { return dominator_; }
   void set_dominator(BasicBlock* dominator);
 
   BasicBlock* loop_header() const { return loop_header_; }
   void set_loop_header(BasicBlock* loop_header);
 
+  BasicBlock* loop_end() const { return loop_end_; }
+  void set_loop_end(BasicBlock* loop_end);
+
   int32_t loop_depth() const { return loop_depth_; }
   void set_loop_depth(int32_t loop_depth);
 
-  int32_t loop_end() const { return loop_end_; }
-  void set_loop_end(int32_t loop_end);
-
   RpoNumber GetAoNumber() const { return RpoNumber::FromInt(ao_number_); }
   int32_t ao_number() const { return ao_number_; }
   void set_ao_number(int32_t ao_number) { ao_number_ = ao_number; }
@@ -166,19 +169,20 @@ class BasicBlock FINAL : public ZoneObject {
   void set_rpo_number(int32_t rpo_number);
 
   // Loop membership helpers.
-  inline bool IsLoopHeader() const { return loop_end_ >= 0; }
+  inline bool IsLoopHeader() const { return loop_end_ != NULL; }
   bool LoopContains(BasicBlock* block) const;
 
  private:
   int32_t ao_number_;        // assembly order number of the block.
   int32_t rpo_number_;       // special RPO number of the block.
   bool deferred_;            // true if the block contains deferred code.
+  int32_t dominator_depth_;  // Depth within the dominator tree.
   BasicBlock* dominator_;    // Immediate dominator of the block.
   BasicBlock* loop_header_;  // Pointer to dominating loop header basic block,
                              // NULL if none. For loop headers, this points to
                              // enclosing loop header.
+  BasicBlock* loop_end_;     // end of the loop, if this block is a loop header.
   int32_t loop_depth_;       // loop nesting, 0 is top-level
-  int32_t loop_end_;         // end of the loop, if this block is a loop header.
 
   Control control_;          // Control at the end of the block.
   Node* control_input_;      // Input value for control.