Use ScopedArenaAllocator for building HGraph.

Memory needed to compile the two most expensive methods for
aosp_angler-userdebug boot image:
  BatteryStats.dumpCheckinLocked() : 21.1MiB -> 20.2MiB
  BatteryStats.dumpLocked(): 42.0MiB -> 40.3MiB
This is because all the memory previously used by the graph
builder is reused by later passes.

And finish the "arena"->"allocator" renaming; make renamed
allocator pointers that are members of classes const when
appropriate (and make a few more members around them const).

Test: m test-art-host-gtest
Test: testrunner.py --host
Bug: 64312607
Change-Id: Ia50aafc80c05941ae5b96984ba4f31ed4c78255e
diff --git a/compiler/utils/assembler_thumb_test.cc b/compiler/utils/assembler_thumb_test.cc
index 5622f89..5307d17 100644
--- a/compiler/utils/assembler_thumb_test.cc
+++ b/compiler/utils/assembler_thumb_test.cc
@@ -167,10 +167,10 @@
 
 class ArmVIXLAssemblerTest : public ::testing::Test {
  public:
-  ArmVIXLAssemblerTest() : pool(), arena(&pool), assembler(&arena) { }
+  ArmVIXLAssemblerTest() : pool(), allocator(&pool), assembler(&allocator) { }
 
   ArenaPool pool;
-  ArenaAllocator arena;
+  ArenaAllocator allocator;
   ArmVIXLJNIMacroAssembler assembler;
 };
 
@@ -209,18 +209,16 @@
   const bool is_critical_native = false;
   const char* shorty = "IIFII";
 
-  ArenaPool pool;
-  ArenaAllocator arena(&pool);
-
   std::unique_ptr<JniCallingConvention> jni_conv(
-      JniCallingConvention::Create(&arena,
+      JniCallingConvention::Create(&allocator,
                                    is_static,
                                    is_synchronized,
                                    is_critical_native,
                                    shorty,
                                    kThumb2));
   std::unique_ptr<ManagedRuntimeCallingConvention> mr_conv(
-      ManagedRuntimeCallingConvention::Create(&arena, is_static, is_synchronized, shorty, kThumb2));
+      ManagedRuntimeCallingConvention::Create(
+          &allocator, is_static, is_synchronized, shorty, kThumb2));
   const int frame_size(jni_conv->FrameSize());
   ArrayRef<const ManagedRegister> callee_save_regs = jni_conv->CalleeSaveRegisters();