Automatic left factoring does not support labels

[sharwell]

The following grammar results in a NullPointerException, due to the use the label op in the incDecStmt rule.

https://github.com/tunnelvisionlabs/goworks/blob/a9549d56969d7d7bcf49816fd1faaed37bfd0e2d/goworks.editor/src/org/tvl/goworks/editor/go/parser/GoParser.g4

Exception in thread "main" java.lang.NullPointerException
    at org.antlr.v4.codegen.model.RuleFunction.addContextDecl(RuleFunction.java:331)
    at org.antlr.v4.codegen.ParserFactory.tokenRef(ParserFactory.java:160)
    at org.antlr.v4.codegen.BlankOutputModelFactory.stringRef(BlankOutputModelFactory.java:88)
    at org.antlr.v4.codegen.OutputModelController.stringRef(OutputModelController.java:403)
    at org.antlr.v4.codegen.SourceGenTriggers.terminal(SourceGenTriggers.java:1581)
    at org.antlr.v4.codegen.SourceGenTriggers.atom(SourceGenTriggers.java:1307)
    at org.antlr.v4.codegen.SourceGenTriggers.blockSet(SourceGenTriggers.java:1066)
    at org.antlr.v4.codegen.SourceGenTriggers.atom(SourceGenTriggers.java:1327)
    at org.antlr.v4.codegen.SourceGenTriggers.labeledElement(SourceGenTriggers.java:828)
    at org.antlr.v4.codegen.SourceGenTriggers.element(SourceGenTriggers.java:594)
    at org.antlr.v4.codegen.SourceGenTriggers.alt(SourceGenTriggers.java:427)
    at org.antlr.v4.codegen.SourceGenTriggers.alternative(SourceGenTriggers.java:335)
    at org.antlr.v4.codegen.SourceGenTriggers.block(SourceGenTriggers.java:270)
    at org.antlr.v4.codegen.OutputModelController.buildNormalRuleFunction(OutputModelController.java:306)
    at org.antlr.v4.codegen.OutputModelController.buildRuleFunction(OutputModelController.java:193)
    at org.antlr.v4.codegen.OutputModelController.buildParserOutputModel(OutputModelController.java:119)
    at org.antlr.v4.codegen.CodeGenerator.generateParser(CodeGenerator.java:156)
    at org.antlr.v4.codegen.CodeGenPipeline.process(CodeGenPipeline.java:82)
    at org.antlr.v4.Tool.processNonCombinedGrammar(Tool.java:426)
    at org.antlr.v4.Tool.process(Tool.java:390)
    at org.antlr.v4.Tool.processGrammarsOnCommandLine(Tool.java:343)
    at org.antlr.v4.Tool.main(Tool.java:190)