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.cc b/compiler/utils/jni_macro_assembler.cc
index 3ac6c3c..0616b35 100644
--- a/compiler/utils/jni_macro_assembler.cc
+++ b/compiler/utils/jni_macro_assembler.cc
@@ -47,7 +47,7 @@
 
 template <>
 MacroAsm32UniquePtr JNIMacroAssembler<PointerSize::k32>::Create(
-    ArenaAllocator* arena,
+    ArenaAllocator* allocator,
     InstructionSet instruction_set,
     const InstructionSetFeatures* instruction_set_features) {
 #ifndef ART_ENABLE_CODEGEN_mips
@@ -58,19 +58,19 @@
 #ifdef ART_ENABLE_CODEGEN_arm
     case kArm:
     case kThumb2:
-      return MacroAsm32UniquePtr(new (arena) arm::ArmVIXLJNIMacroAssembler(arena));
+      return MacroAsm32UniquePtr(new (allocator) arm::ArmVIXLJNIMacroAssembler(allocator));
 #endif
 #ifdef ART_ENABLE_CODEGEN_mips
     case kMips:
-      return MacroAsm32UniquePtr(new (arena) mips::MipsAssembler(
-          arena,
+      return MacroAsm32UniquePtr(new (allocator) mips::MipsAssembler(
+          allocator,
           instruction_set_features != nullptr
               ? instruction_set_features->AsMipsInstructionSetFeatures()
               : nullptr));
 #endif
 #ifdef ART_ENABLE_CODEGEN_x86
     case kX86:
-      return MacroAsm32UniquePtr(new (arena) x86::X86JNIMacroAssembler(arena));
+      return MacroAsm32UniquePtr(new (allocator) x86::X86JNIMacroAssembler(allocator));
 #endif
     default:
       LOG(FATAL) << "Unknown/unsupported 4B InstructionSet: " << instruction_set;
@@ -82,7 +82,7 @@
 
 template <>
 MacroAsm64UniquePtr JNIMacroAssembler<PointerSize::k64>::Create(
-    ArenaAllocator* arena,
+    ArenaAllocator* allocator,
     InstructionSet instruction_set,
     const InstructionSetFeatures* instruction_set_features) {
 #ifndef ART_ENABLE_CODEGEN_mips64
@@ -92,22 +92,22 @@
   switch (instruction_set) {
 #ifdef ART_ENABLE_CODEGEN_arm64
     case kArm64:
-      return MacroAsm64UniquePtr(new (arena) arm64::Arm64JNIMacroAssembler(arena));
+      return MacroAsm64UniquePtr(new (allocator) arm64::Arm64JNIMacroAssembler(allocator));
 #endif
 #ifdef ART_ENABLE_CODEGEN_mips64
     case kMips64:
-      return MacroAsm64UniquePtr(new (arena) mips64::Mips64Assembler(
-          arena,
+      return MacroAsm64UniquePtr(new (allocator) mips64::Mips64Assembler(
+          allocator,
           instruction_set_features != nullptr
               ? instruction_set_features->AsMips64InstructionSetFeatures()
               : nullptr));
 #endif
 #ifdef ART_ENABLE_CODEGEN_x86_64
     case kX86_64:
-      return MacroAsm64UniquePtr(new (arena) x86_64::X86_64JNIMacroAssembler(arena));
+      return MacroAsm64UniquePtr(new (allocator) x86_64::X86_64JNIMacroAssembler(allocator));
 #endif
     default:
-      UNUSED(arena);
+      UNUSED(allocator);
       LOG(FATAL) << "Unknown/unsupported 8B InstructionSet: " << instruction_set;
       UNREACHABLE();
   }