Use symbolic rather than duplicate string for pass.

Test: none

Change-Id: Ia912d75e72e7002d098db80ec88f82775d88f6cc
diff --git a/compiler/optimizing/instruction_simplifier_mips.h b/compiler/optimizing/instruction_simplifier_mips.h
index 22cc2ef..6cb8aff 100644
--- a/compiler/optimizing/instruction_simplifier_mips.h
+++ b/compiler/optimizing/instruction_simplifier_mips.h
@@ -30,7 +30,7 @@
 class InstructionSimplifierMips : public HOptimization {
  public:
   InstructionSimplifierMips(HGraph* graph, CodeGenerator* codegen, OptimizingCompilerStats* stats)
-      : HOptimization(graph, "instruction_simplifier_mips", stats),
+      : HOptimization(graph, kInstructionSimplifierMipsPassName, stats),
         codegen_(down_cast<CodeGeneratorMIPS*>(codegen)) {}
 
   static constexpr const char* kInstructionSimplifierMipsPassName = "instruction_simplifier_mips";