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/mips/assembler_mips.h b/compiler/utils/mips/assembler_mips.h
index e82693a..57b3edd 100644
--- a/compiler/utils/mips/assembler_mips.h
+++ b/compiler/utils/mips/assembler_mips.h
@@ -192,16 +192,16 @@
  public:
   using JNIBase = JNIMacroAssembler<PointerSize::k32>;
 
-  explicit MipsAssembler(ArenaAllocator* arena,
+  explicit MipsAssembler(ArenaAllocator* allocator,
                          const MipsInstructionSetFeatures* instruction_set_features = nullptr)
-      : Assembler(arena),
+      : Assembler(allocator),
         overwriting_(false),
         overwrite_location_(0),
         reordering_(true),
         ds_fsm_state_(kExpectingLabel),
         ds_fsm_target_pc_(0),
-        literals_(arena->Adapter(kArenaAllocAssembler)),
-        jump_tables_(arena->Adapter(kArenaAllocAssembler)),
+        literals_(allocator->Adapter(kArenaAllocAssembler)),
+        jump_tables_(allocator->Adapter(kArenaAllocAssembler)),
         last_position_adjustment_(0),
         last_old_position_(0),
         last_branch_id_(0),
diff --git a/compiler/utils/mips/assembler_mips32r5_test.cc b/compiler/utils/mips/assembler_mips32r5_test.cc
index a3662db..9a69ffd 100644
--- a/compiler/utils/mips/assembler_mips32r5_test.cc
+++ b/compiler/utils/mips/assembler_mips32r5_test.cc
@@ -72,8 +72,8 @@
     return " -D -bbinary -mmips:isa32r5";
   }
 
-  mips::MipsAssembler* CreateAssembler(ArenaAllocator* arena) OVERRIDE {
-    return new (arena) mips::MipsAssembler(arena, instruction_set_features_.get());
+  mips::MipsAssembler* CreateAssembler(ArenaAllocator* allocator) OVERRIDE {
+    return new (allocator) mips::MipsAssembler(allocator, instruction_set_features_.get());
   }
 
   void SetUpHelpers() OVERRIDE {
diff --git a/compiler/utils/mips/assembler_mips32r6_test.cc b/compiler/utils/mips/assembler_mips32r6_test.cc
index b6cb30a..b12b6b6 100644
--- a/compiler/utils/mips/assembler_mips32r6_test.cc
+++ b/compiler/utils/mips/assembler_mips32r6_test.cc
@@ -85,8 +85,8 @@
     return " -D -bbinary -mmips:isa32r6";
   }
 
-  mips::MipsAssembler* CreateAssembler(ArenaAllocator* arena) OVERRIDE {
-    return new (arena) mips::MipsAssembler(arena, instruction_set_features_.get());
+  mips::MipsAssembler* CreateAssembler(ArenaAllocator* allocator) OVERRIDE {
+    return new (allocator) mips::MipsAssembler(allocator, instruction_set_features_.get());
   }
 
   void SetUpHelpers() OVERRIDE {