Move two members out of MIRGraph::temp_::ssa.

It turns out they are used outside the SSA transformation
by the x86 back-end.

This is a partial revert of
  https://android-review.googlesource.com/120571

Change-Id: Ia5cb2988ab0625d8519901124bd4fc184d5f0886
diff --git a/compiler/dex/mir_graph.cc b/compiler/dex/mir_graph.cc
index 8b73863..312a6eb 100644
--- a/compiler/dex/mir_graph.cc
+++ b/compiler/dex/mir_graph.cc
@@ -82,6 +82,8 @@
       cu_(cu),
       ssa_base_vregs_(arena->Adapter(kArenaAllocSSAToDalvikMap)),
       ssa_subscripts_(arena->Adapter(kArenaAllocSSAToDalvikMap)),
+      vreg_to_ssa_map_(NULL),
+      ssa_last_defs_(NULL),
       is_constant_v_(NULL),
       constant_values_(NULL),
       use_counts_(arena->Adapter()),