ART: Add -Wunused

Until the global CFLAGS are fixed, add Wunused. Fix declarations
in the optimizing compiler.

Change-Id: Ic4553f08e809dc54f3d82af57ac592622c98e000
diff --git a/compiler/optimizing/instruction_simplifier.h b/compiler/optimizing/instruction_simplifier.h
index a7ff755..0244620 100644
--- a/compiler/optimizing/instruction_simplifier.h
+++ b/compiler/optimizing/instruction_simplifier.h
@@ -30,9 +30,11 @@
  public:
   InstructionSimplifier(HGraph* graph,
                         OptimizingCompilerStats* stats = nullptr,
-                        const char* name = "instruction_simplifier")
+                        const char* name = kInstructionSimplifierPassName)
     : HOptimization(graph, true, name, stats) {}
 
+  static constexpr const char* kInstructionSimplifierPassName = "instruction_simplifier";
+
   void Run() OVERRIDE;
 };