Remove ActAsNullConstant

We now properly type null constants during ssa builder so this is not
needed anymore.

Bug: 20322006
Change-Id: Ic060a52d4fa2d4f00755dd6427f822d368392d7b
diff --git a/compiler/optimizing/reference_type_propagation.cc b/compiler/optimizing/reference_type_propagation.cc
index de6941c..12b1c2b 100644
--- a/compiler/optimizing/reference_type_propagation.cc
+++ b/compiler/optimizing/reference_type_propagation.cc
@@ -68,11 +68,11 @@
   }
   HInstruction* input0 = ifInput->InputAt(0);
   HInstruction* input1 = ifInput->InputAt(1);
-  HInstruction* obj;
+  HInstruction* obj = nullptr;
 
-  if ((input0->GetType() == Primitive::kPrimNot) && input1->ActAsNullConstant()) {
+  if (input1->IsNullConstant()) {
     obj = input0;
-  } else if ((input1->GetType() == Primitive::kPrimNot) && input0->ActAsNullConstant()) {
+  } else if (input0->IsNullConstant()) {
     obj = input1;
   } else {
     return;