Opt compiler: Clean the use of `virtual` and `OVERRIDE`.

Change-Id: I806ec522b979334cee8f344fc95e8660c019160a
diff --git a/compiler/optimizing/ssa_test.cc b/compiler/optimizing/ssa_test.cc
index 7fc1ec6..a05b38c 100644
--- a/compiler/optimizing/ssa_test.cc
+++ b/compiler/optimizing/ssa_test.cc
@@ -32,15 +32,15 @@
  public:
   explicit SsaPrettyPrinter(HGraph* graph) : HPrettyPrinter(graph), str_("") {}
 
-  virtual void PrintInt(int value) {
+  void PrintInt(int value) OVERRIDE {
     str_ += StringPrintf("%d", value);
   }
 
-  virtual void PrintString(const char* value) {
+  void PrintString(const char* value) OVERRIDE {
     str_ += value;
   }
 
-  virtual void PrintNewLine() {
+  void PrintNewLine() OVERRIDE {
     str_ += '\n';
   }
 
@@ -48,7 +48,7 @@
 
   std::string str() const { return str_; }
 
-  virtual void VisitIntConstant(HIntConstant* constant) {
+  void VisitIntConstant(HIntConstant* constant) OVERRIDE {
     PrintPreInstruction(constant);
     str_ += constant->DebugName();
     str_ += " ";