ART: Replace NULL to nullptr in the optimizing compiler

Replace macro NULL to the nullptr variation for C++.

Change-Id: Ib6e48dd4bb3c254343383011b67372622578ca76
Signed-off-by: Jean Christophe Beyler <jean.christophe.beyler@intel.com>
diff --git a/compiler/optimizing/code_generator.cc b/compiler/optimizing/code_generator.cc
index 6f424ce..5f7a121 100644
--- a/compiler/optimizing/code_generator.cc
+++ b/compiler/optimizing/code_generator.cc
@@ -369,7 +369,7 @@
     uint32_t native_offset = pc_info.native_pc;
     uint32_t dex_pc = pc_info.dex_pc;
     const uint8_t* references = dex_gc_map.FindBitMap(dex_pc, false);
-    CHECK(references != NULL) << "Missing ref for dex pc 0x" << std::hex << dex_pc;
+    CHECK(references != nullptr) << "Missing ref for dex pc 0x" << std::hex << dex_pc;
     builder.AddEntry(native_offset, references);
   }
 }
diff --git a/compiler/optimizing/graph_visualizer.cc b/compiler/optimizing/graph_visualizer.cc
index 5d1703e..99b5219 100644
--- a/compiler/optimizing/graph_visualizer.cc
+++ b/compiler/optimizing/graph_visualizer.cc
@@ -66,7 +66,7 @@
 
   void PrintTime(const char* name) {
     AddIndent();
-    output_ << name << " " << time(NULL) << std::endl;
+    output_ << name << " " << time(nullptr) << std::endl;
   }
 
   void PrintInt(const char* name, int value) {
diff --git a/compiler/optimizing/nodes.cc b/compiler/optimizing/nodes.cc
index fb941b5..bcd3f0a 100644
--- a/compiler/optimizing/nodes.cc
+++ b/compiler/optimizing/nodes.cc
@@ -445,7 +445,7 @@
   HUseListNode<T>* current = *list;
   while (current != nullptr) {
     if (current->GetUser() == user && current->GetIndex() == input_index) {
-      if (previous == NULL) {
+      if (previous == nullptr) {
         *list = current->GetTail();
       } else {
         previous->SetTail(current->GetTail());