[turbofan] Introduce conditional builders into AstGraphBuilder.

src/compiler/ast-graph-builder.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_AST_GRAPH_BUILDER_H_
 #define V8_COMPILER_AST_GRAPH_BUILDER_H_
 
 #include "src/ast.h"
 #include "src/compiler/js-graph.h"
 #include "src/compiler/liveness-analyzer.h"
@@ skipping 337 matching lines @@
   Node* BuildThrow(Node* exception_value);
 
   // Builders for binary operations.
   Node* BuildBinaryOp(Node* left, Node* right, Token::Value op);
 
   // Process arguments to a call by popping {arity} elements off the operand
   // stack and build a call node using the given call operator.
   Node* ProcessArguments(const Operator* op, int arity);
 
   // ===========================================================================
+  // The following build methods have the same contract as the above ones, but
+  // they can also return {NULL} to indicate that no fragment was built. Note
+  // that these are optimizations, disabling any of them should still produce
+  // correct graphs.
+
+  // Optimization for variable load from global object.
+  Node* TryLoadGlobalConstant(Handle<Name> name);
+
+  // Optimizations for automatic type conversion.
+  Node* TryFastToBoolean(Node* input);
+  Node* TryFastToName(Node* input);
+
+  // ===========================================================================
   // The following visitation methods all recursively visit a subtree of the
   // underlying AST and extent the graph. The operand stack is mutated in a way
   // consistent with other compilers:
   //  - Expressions pop operands and push result, depending on {AstContext}.
   //  - Statements keep the operand stack balanced.
 
   // Visit statements.
   void VisitIfNotNull(Statement* stmt);
   void VisitInScope(Statement* stmt, Scope* scope, Node* context);
 
@@ skipping 185 matching lines @@
 
   // Prepare environment to be used as loop header.
   void PrepareForLoop(BitVector* assigned, bool is_osr = false);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_AST_GRAPH_BUILDER_H_