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/x86/jni_macro_assembler_x86.h b/compiler/utils/x86/jni_macro_assembler_x86.h
index 75cdd1e..56eaf19 100644
--- a/compiler/utils/x86/jni_macro_assembler_x86.h
+++ b/compiler/utils/x86/jni_macro_assembler_x86.h
@@ -34,7 +34,7 @@
 
 class X86JNIMacroAssembler FINAL : public JNIMacroAssemblerFwd<X86Assembler, PointerSize::k32> {
  public:
-  explicit X86JNIMacroAssembler(ArenaAllocator* arena) : JNIMacroAssemblerFwd(arena) {}
+  explicit X86JNIMacroAssembler(ArenaAllocator* allocator) : JNIMacroAssemblerFwd(allocator) {}
   virtual ~X86JNIMacroAssembler() {}
 
   //