[turbofan]: Add a context relaxation Reducer

src/compiler/ast-graph-builder.h

Index: src/compiler/ast-graph-builder.h
diff --git a/src/compiler/ast-graph-builder.h b/src/compiler/ast-graph-builder.h
index 770d2d94345f41972f30df9095fbee428191f142..bbde90f10e637f9a4a20de933641491130eba919 100644
--- a/src/compiler/ast-graph-builder.h
+++ b/src/compiler/ast-graph-builder.h
@@ -52,6 +52,8 @@ class AstGraphBuilder : public AstVisitor {
   AST_NODE_LIST(DECLARE_VISIT)
 #undef DECLARE_VISIT
 
+  Node* relaxed_context() const { return relaxed_context_; }
+
   // Visiting function for declarations list is overridden.
   void VisitDeclarations(ZoneList<Declaration*>* declarations) override;
 
@@ -118,6 +120,10 @@ class AstGraphBuilder : public AstVisitor {
   // Type feedback table.
   JSTypeFeedbackTable* js_type_feedback_;
 
+  // The context that can be used for context relaxation if possible, otherwise
+  // nullptr.
+  Node* relaxed_context_;
+
   // Growth increment for the temporary buffer used to construct input lists to
   // new nodes.
   static const int kInputBufferSizeIncrement = 64;