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_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 {