added constant folding & branch elimination to skslc

src/sksl/SkSLIRGenerator.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
  
 #ifndef SKSL_IRGENERATOR
 #define SKSL_IRGENERATOR
 
@@ skipping 71 matching lines @@
     std::unique_ptr<Expression> coerce(std::unique_ptr<Expression> expr, const Type& type);
     std::unique_ptr<Block> convertBlock(const ASTBlock& block);
     std::unique_ptr<Statement> convertBreak(const ASTBreakStatement& b);
     std::unique_ptr<Expression> convertConstructor(Position position, 
                                                    const Type& type, 
                                                    std::vector<std::unique_ptr<Expression>> params);
     std::unique_ptr<Statement> convertContinue(const ASTContinueStatement& c);
     std::unique_ptr<Statement> convertDiscard(const ASTDiscardStatement& d);
     std::unique_ptr<Statement> convertDo(const ASTDoStatement& d);
     std::unique_ptr<Expression> convertBinaryExpression(const ASTBinaryExpression& expression);
+    std::unique_ptr<Expression> constantFold(const Expression& left,

[dogben, 2016/11/09 15:35:36]

nit: Should probably document that returning nullptr does not indicate a
compilation error.

+                                             Token::Kind op,
+                                             const Expression& right);
     std::unique_ptr<Extension> convertExtension(const ASTExtension& e);
     std::unique_ptr<Statement> convertExpressionStatement(const ASTExpressionStatement& s);
     std::unique_ptr<Statement> convertFor(const ASTForStatement& f);
     std::unique_ptr<Expression> convertIdentifier(const ASTIdentifier& identifier);
     std::unique_ptr<Statement> convertIf(const ASTIfStatement& s);
     std::unique_ptr<Expression> convertIndex(std::unique_ptr<Expression> base,
                                              const ASTExpression& index);
     std::unique_ptr<InterfaceBlock> convertInterfaceBlock(const ASTInterfaceBlock& s);
     Modifiers convertModifiers(const ASTModifiers& m);
     std::unique_ptr<Expression> convertPrefixExpression(const ASTPrefixExpression& expression);
@@ skipping 18 matching lines @@
     ErrorReporter& fErrors;
 
     friend class AutoSymbolTable;
     friend class AutoLoopLevel;
     friend class Compiler;
 };
 
 }
 
 #endif