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/mips64/assembler_mips64.h b/compiler/utils/mips64/assembler_mips64.h
index ee78cdb..a3787ac 100644
--- a/compiler/utils/mips64/assembler_mips64.h
+++ b/compiler/utils/mips64/assembler_mips64.h
@@ -418,14 +418,14 @@
  public:
   using JNIBase = JNIMacroAssembler<PointerSize::k64>;
 
-  explicit Mips64Assembler(ArenaAllocator* arena,
+  explicit Mips64Assembler(ArenaAllocator* allocator,
                            const Mips64InstructionSetFeatures* instruction_set_features = nullptr)
-      : Assembler(arena),
+      : Assembler(allocator),
         overwriting_(false),
         overwrite_location_(0),
-        literals_(arena->Adapter(kArenaAllocAssembler)),
-        long_literals_(arena->Adapter(kArenaAllocAssembler)),
-        jump_tables_(arena->Adapter(kArenaAllocAssembler)),
+        literals_(allocator->Adapter(kArenaAllocAssembler)),
+        long_literals_(allocator->Adapter(kArenaAllocAssembler)),
+        jump_tables_(allocator->Adapter(kArenaAllocAssembler)),
         last_position_adjustment_(0),
         last_old_position_(0),
         last_branch_id_(0),