Disable DCE when there are irreducible loops.

Also ensure an instruction that requires an environment
does have one.

Change-Id: I41a8460e05ef320f872197d3be7847e7ffaa6ee8
diff --git a/compiler/optimizing/nodes.h b/compiler/optimizing/nodes.h
index 859d570..e222ef7 100644
--- a/compiler/optimizing/nodes.h
+++ b/compiler/optimizing/nodes.h
@@ -860,6 +860,8 @@
   HInstruction* GetLastPhi() const { return phis_.last_instruction_; }
   const HInstructionList& GetPhis() const { return phis_; }
 
+  HInstruction* GetFirstInstructionDisregardMoves() const;
+
   void AddSuccessor(HBasicBlock* block) {
     successors_.push_back(block);
     block->predecessors_.push_back(this);