Quick: Rewrite Phi node insertion.

Delay Phi node insertion to the SSAConversion pass to allow
updating the vreg_to_ssa_map_ with INVALID_SREG when we omit
a Phi in the pruned SSA form.

Change-Id: I450dee21f7dc4353d25fc66f4d0ee01671de6e0e
diff --git a/compiler/dex/mir_graph.cc b/compiler/dex/mir_graph.cc
index 93a31e9..92f960e 100644
--- a/compiler/dex/mir_graph.cc
+++ b/compiler/dex/mir_graph.cc
@@ -1798,7 +1798,8 @@
 
   temp_.ssa.num_vregs = 0u;
   temp_.ssa.work_live_vregs = nullptr;
-  temp_.ssa.def_block_matrix = nullptr;
+  DCHECK(temp_.ssa.def_block_matrix == nullptr);
+  temp_.ssa.phi_node_blocks = nullptr;
   DCHECK(temp_scoped_alloc_.get() != nullptr);
   temp_scoped_alloc_.reset();