fix bug in optimization: only kill the *entirety* of `yyvstack` value tracking in the parser kernel when none of the parser rules use it, i.e. there's no explicit or *implicit* `$$` / `$N` value assignment/access anywhere. The 'implicit' part is where things went wrong because a grammar which DOES NOT also have the `--no-default-action` option set and has no action code for any of its rules *still* executes the *implicit* `$$ = $1` actions in the kernel, hence killing all of that as well is an error as then the parser kernel will behave differently!
fix bug in optimization: only kill the *entirety* of `yyvstack` value tracking in the parser kernel when none of the parser rules use it, i.e. there's no explicit or *implicit* `$$` / `$N` value assignment/access anywhere. The 'implicit' part is where things went wrong because a grammar which DOES NOT also have the `--no-default-action` option set and has no action code for any of its rules *still* executes the *implicit* `$$ = $1` actions in the kernel, hence killing all of that as well is an error as then the parser kernel will behave differently!

GerHobbelt/jison,GerHobbelt/jison,GerHobbelt/jison,GerHobbelt/jison