Move Assemblers to the Arena.

And clean up some APIs to return std::unique_ptr<> instead
of raw pointers that don't communicate ownership.

Change-Id: I3017302307a0253d661240750298802fb0d9585e
diff --git a/compiler/optimizing/code_generator_x86_64.cc b/compiler/optimizing/code_generator_x86_64.cc
index 056b69b..59cc444 100644
--- a/compiler/optimizing/code_generator_x86_64.cc
+++ b/compiler/optimizing/code_generator_x86_64.cc
@@ -1007,6 +1007,7 @@
         location_builder_(graph, this),
         instruction_visitor_(graph, this),
         move_resolver_(graph->GetArena(), this),
+        assembler_(graph->GetArena()),
         isa_features_(isa_features),
         constant_area_start_(0),
         method_patches_(graph->GetArena()->Adapter(kArenaAllocCodeGenerator)),