[turbofan]: Add a context relaxer reducer

src/compiler/js-context-relaxation.h

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_COMPILER_JS_CONTEXT_RELAXATION_H_
+#define V8_COMPILER_JS_CONTEXT_RELAXATION_H_
+
+#include "src/compiler/graph-reducer.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+// Ensures that operations that only need to access the native context use the
+// outer-most context rather than the specific context given by the AST graph
+// builder. This makes it possible to use these operations with context
+// specialization (e.g. for generating stubs) without forcing inner contexts to
+// be embedded in generated code thus causing leaks and potentially using the
+// wrong native context (i.e. stubs are shared between native contexts).
+class JSContextRelaxation final : public Reducer {
+ public:
+  JSContextRelaxation() {}
+  ~JSContextRelaxation() final {}
+
+  Reduction Reduce(Node* node) final;
+};
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_COMPILER_JS_CONTEXT_RELAXATION_H_