[turbofan] Introduce conditional builders into AstGraphBuilder.

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 8315e9cd11e9241141c5052cb6b37195a54708c7..d2e77e56dbc2715ad83fb20701d08ae5cb1e345a 100644
--- a/src/compiler/ast-graph-builder.h
+++ b/src/compiler/ast-graph-builder.h
@@ -355,6 +355,19 @@ class AstGraphBuilder : public AstVisitor {
   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: