Fix a bug in optimizing when the null constant has been DCE.

If it has been DCE, we should create a new one, instead of
using the old one.

Also move the first DCE to a place where it could actually
be useful.

bug:21870788

Change-Id: I28fc52ae481ef92cba45fc1b5abcf07c995f524c
diff --git a/compiler/optimizing/optimizing_compiler.cc b/compiler/optimizing/optimizing_compiler.cc
index 303a7cb..c695abe 100644
--- a/compiler/optimizing/optimizing_compiler.cc
+++ b/compiler/optimizing/optimizing_compiler.cc
@@ -348,10 +348,10 @@
 
   HOptimization* optimizations[] = {
     intrinsics,
-    dce1,
     fold1,
     simplify1,
     type_propagation,
+    dce1,
     simplify2,
     inliner,
     // Run another type propagation phase: inlining will open up more opprotunities