added constant folding & branch elimination to skslc

src/sksl/SkSLIRGenerator.h

Index: src/sksl/SkSLIRGenerator.h
diff --git a/src/sksl/SkSLIRGenerator.h b/src/sksl/SkSLIRGenerator.h
index a1b86f4f54f6889ca6ad6c6a16ca475239e93c0e..ddcfff82320357179e44163f5bc986528f86f4cf 100644
--- a/src/sksl/SkSLIRGenerator.h
+++ b/src/sksl/SkSLIRGenerator.h
@@ -89,6 +89,9 @@ private:
     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);