Use ScopedArenaAllocator for register allocation.

Memory needed to compile the two most expensive methods for
aosp_angler-userdebug boot image:
  BatteryStats.dumpCheckinLocked() : 25.1MiB -> 21.1MiB
  BatteryStats.dumpLocked(): 49.6MiB -> 42.0MiB
This is because all the memory previously used by Scheduler
is reused by the register allocator; the register allocator
has a higher peak usage of the ArenaStack.

And continue the "arena"->"allocator" renaming.

Test: m test-art-host-gtest
Test: testrunner.py --host
Bug: 64312607
Change-Id: Idfd79a9901552b5147ec0bf591cb38120de86b01
diff --git a/compiler/utils/jni_macro_assembler.h b/compiler/utils/jni_macro_assembler.h
index 72f1ce0..0fc1353 100644
--- a/compiler/utils/jni_macro_assembler.h
+++ b/compiler/utils/jni_macro_assembler.h
@@ -46,7 +46,7 @@
 class JNIMacroAssembler : public DeletableArenaObject<kArenaAllocAssembler> {
  public:
   static std::unique_ptr<JNIMacroAssembler<kPointerSize>> Create(
-      ArenaAllocator* arena,
+      ArenaAllocator* allocator,
       InstructionSet instruction_set,
       const InstructionSetFeatures* instruction_set_features = nullptr);
 
@@ -275,7 +275,7 @@
   }
 
  protected:
-  explicit JNIMacroAssemblerFwd(ArenaAllocator* arena) : asm_(arena) {}
+  explicit JNIMacroAssemblerFwd(ArenaAllocator* allocator) : asm_(allocator) {}
 
   T asm_;
 };