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_mips.cc b/compiler/optimizing/code_generator_mips.cc
index 185397c..12d1164 100644
--- a/compiler/optimizing/code_generator_mips.cc
+++ b/compiler/optimizing/code_generator_mips.cc
@@ -471,7 +471,7 @@
       location_builder_(graph, this),
       instruction_visitor_(graph, this),
       move_resolver_(graph->GetArena(), this),
-      assembler_(&isa_features),
+      assembler_(graph->GetArena(), &isa_features),
       isa_features_(isa_features) {
   // Save RA (containing the return address) to mimic Quick.
   AddAllocatedRegister(Location::RegisterLocation(RA));