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/x86/assembler_x86_test.cc b/compiler/utils/x86/assembler_x86_test.cc
index cccde37..e232add 100644
--- a/compiler/utils/x86/assembler_x86_test.cc
+++ b/compiler/utils/x86/assembler_x86_test.cc
@@ -24,8 +24,8 @@
 
 TEST(AssemblerX86, CreateBuffer) {
   ArenaPool pool;
-  ArenaAllocator arena(&pool);
-  AssemblerBuffer buffer(&arena);
+  ArenaAllocator allocator(&pool);
+  AssemblerBuffer buffer(&allocator);
   AssemblerBuffer::EnsureCapacity ensured(&buffer);
   buffer.Emit<uint8_t>(0x42);
   ASSERT_EQ(static_cast<size_t>(1), buffer.Size());