makeForInOfLeft() {
    const context = this.loopContext;
    const forkContext = this.forkContext;
    const leftSegments = forkContext.makeDisconnected(-1, -1);

    // Update state.
    context.prevSegments = forkContext.head;
    context.leftSegments = context.continueDestSegments = leftSegments;
    forkContext.replaceHead(leftSegments);
  }
Makes a code path segment for the left part of a ForInStatement and a
ForOfStatement.
@returns {void}