Mark graph visitor classes as final.

This should improve optimization opportunities for clang++.

Test: buildbot-build.sh
Change-Id: Ib0c1ebfb157176a9063cca2ed465cff6fb280442
diff --git a/compiler/optimizing/instruction_simplifier.cc b/compiler/optimizing/instruction_simplifier.cc
index b772fbb..bf51b5d 100644
--- a/compiler/optimizing/instruction_simplifier.cc
+++ b/compiler/optimizing/instruction_simplifier.cc
@@ -37,7 +37,7 @@
 // is replaced with its copy if it is clonable.
 static constexpr bool kTestInstructionClonerExhaustively = false;
 
-class InstructionSimplifierVisitor : public HGraphDelegateVisitor {
+class InstructionSimplifierVisitor final : public HGraphDelegateVisitor {
  public:
   InstructionSimplifierVisitor(HGraph* graph,
                                CodeGenerator* codegen,