Make CTX allocatable by the register allocator.

runtime/vm/flow_graph.h

Index: runtime/vm/flow_graph.h
===================================================================
--- runtime/vm/flow_graph.h	(revision 41393)
+++ runtime/vm/flow_graph.h	(working copy)
@@ -111,6 +111,15 @@
     return num_non_copied_params_;
   }
 
+  LocalVariable* CurrentContextVar() const {
+    return parsed_function().current_context_var();
+  }
+
+  intptr_t CurrentContextEnvIndex() const {
+    return parsed_function().current_context_var()->BitIndexIn(
+        num_non_copied_params_);
+  }
+
   // Flow graph orders.
   const GrowableArray<BlockEntryInstr*>& preorder() const {
     return preorder_;
@@ -159,6 +168,10 @@
     return constant_dead_;
   }
 
+  ConstantInstr* constant_empty_context() const {
+    return constant_empty_context_;
+  }
+
   intptr_t alloc_ssa_temp_index() { return current_ssa_temp_index_++; }
 
   void AllocateSSAIndexes(Definition* def) {
@@ -296,7 +309,8 @@
   void InsertPhis(
       const GrowableArray<BlockEntryInstr*>& preorder,
       const GrowableArray<BitVector*>& assigned_vars,
-      const GrowableArray<BitVector*>& dom_frontier);
+      const GrowableArray<BitVector*>& dom_frontier,
+      GrowableArray<PhiInstr*>* live_phis);
 
   void RemoveDeadPhis(GrowableArray<PhiInstr*>* live_phis);
 
@@ -333,6 +347,7 @@
   GrowableArray<BlockEntryInstr*> optimized_block_order_;
   ConstantInstr* constant_null_;
   ConstantInstr* constant_dead_;
+  ConstantInstr* constant_empty_context_;
 
   BlockEffects* block_effects_;
   bool licm_allowed_;