Create a scoped arena allocator and use that for LVN.

This saves more than 0.5s of boot.oat compilation time
on Nexus 5.

TODO: Move other stuff to the scoped allocator. This CL
alone increases the peak memory allocation. By reusing
the memory for other parts of the compilation we should
reduce this overhead.

Change-Id: Ifbc00aab4f3afd0000da818dfe68b96713824a08
diff --git a/compiler/dex/quick/mir_to_lir.h b/compiler/dex/quick/mir_to_lir.h
index 9e0e299..6955577 100644
--- a/compiler/dex/quick/mir_to_lir.h
+++ b/compiler/dex/quick/mir_to_lir.h
@@ -311,7 +311,7 @@
       virtual void Compile() = 0;
 
       static void* operator new(size_t size, ArenaAllocator* arena) {
-        return arena->Alloc(size, ArenaAllocator::kAllocData);
+        return arena->Alloc(size, kArenaAllocData);
       }
 
      protected:
@@ -363,7 +363,7 @@
     // strdup(), but allocates from the arena.
     char* ArenaStrdup(const char* str) {
       size_t len = strlen(str) + 1;
-      char* res = reinterpret_cast<char*>(arena_->Alloc(len, ArenaAllocator::kAllocMisc));
+      char* res = reinterpret_cast<char*>(arena_->Alloc(len, kArenaAllocMisc));
       if (res != NULL) {
         strncpy(res, str, len);
       }