Allocate code generators on the arena.

Change-Id: If8cf0ee43711f6e13171443e3c057ff370ccfbaa
diff --git a/compiler/optimizing/optimizing_cfi_test.cc b/compiler/optimizing/optimizing_cfi_test.cc
index 2b0d522..400686d 100644
--- a/compiler/optimizing/optimizing_cfi_test.cc
+++ b/compiler/optimizing/optimizing_cfi_test.cc
@@ -54,7 +54,7 @@
     isa_features_.reset(InstructionSetFeatures::FromVariant(isa, "default", &error));
     graph_ = CreateGraph(&allocator_);
     // Generate simple frame with some spills.
-    code_gen_.reset(CodeGenerator::Create(graph_, isa, *isa_features_, opts_));
+    code_gen_ = CodeGenerator::Create(graph_, isa, *isa_features_, opts_);
     code_gen_->GetAssembler()->cfi().SetEnabled(true);
     const int frame_size = 64;
     int core_reg = 0;