Add support for XMM registers in SSA code generation pipeline.

runtime/vm/flow_graph.h

Index: runtime/vm/flow_graph.h
diff --git a/runtime/vm/flow_graph.h b/runtime/vm/flow_graph.h
index e76dd22c4cb71da0bf0e9dba2d36e9b0f2c5b098..b1f396931bab56f85eefd65bd89fd155417d5fe8 100644
--- a/runtime/vm/flow_graph.h
+++ b/runtime/vm/flow_graph.h
@@ -56,6 +56,8 @@ class FlowGraph: public ZoneAllocated {
     return current_ssa_temp_index();
   }
 
+  intptr_t alloc_ssa_temp_index() { return current_ssa_temp_index_++; }
+
   // Operations on the flow graph.
   void ComputeSSA();
 
@@ -91,7 +93,6 @@ class FlowGraph: public ZoneAllocated {
   void MarkLivePhis(GrowableArray<PhiInstr*>* live_phis);
 
   intptr_t current_ssa_temp_index() const { return current_ssa_temp_index_; }
-  intptr_t alloc_ssa_temp_index() { return current_ssa_temp_index_++; }
 
   // DiscoverBlocks computes parent_ and assigned_vars_ which are then used
   // if/when computing SSA.