Implement DoWhile.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
===================================================================
--- runtime/vm/intermediate_language.h	(revision 5046)
+++ runtime/vm/intermediate_language.h	(working copy)
@@ -711,11 +711,11 @@
   DECLARE_INSTRUCTION(Branch)
 
   Value* value() const { return value_; }
-  TargetEntryInstr* true_successor() const { return true_successor_; }
-  TargetEntryInstr* false_successor() const { return false_successor_; }
+  BlockEntryInstr* true_successor() const { return true_successor_; }
+  BlockEntryInstr* false_successor() const { return false_successor_; }
 
-  TargetEntryInstr** true_successor_address() { return &true_successor_; }
-  TargetEntryInstr** false_successor_address() { return &false_successor_; }
+  BlockEntryInstr** true_successor_address() { return &true_successor_; }
+  BlockEntryInstr** false_successor_address() { return &false_successor_; }
 
   virtual void SetSuccessor(Instruction* instr) { UNREACHABLE(); }
 
@@ -723,8 +723,8 @@
 
  private:
   Value* value_;
-  TargetEntryInstr* true_successor_;
-  TargetEntryInstr* false_successor_;
+  BlockEntryInstr* true_successor_;
+  BlockEntryInstr* false_successor_;
 
   DISALLOW_COPY_AND_ASSIGN(BranchInstr);
 };