recast.print: modifying a trailing CommentBlock when returning an identifier leads to assertion error

I think I've got it down to a fairly minimal test case.

Running the following code...

```javascript
'use strict';

const recast = require('recast');

const ast = recast.parse(`function a() {
return x/*comment*/;
}`);

recast.types.visit(ast, {
visitComment: function (path) {
path.value.value = path.value.value.replace('comment', 'COMMENT');
this.traverse(path);
}
});

console.log(recast.print(ast).code);
```

leads to this assertion error:

```
AssertionError: false == true
at pushSlice (./node_modules/recast/lib/patcher.js:47:14)
at Patcher.self.get (./node_modules/recast/lib/patcher.js:63:5)
at ./node_modules/recast/lib/patcher.js:218:20
at maybeReprint (./node_modules/recast/lib/printer.js:95:41)
at print (./node_modules/recast/lib/printer.js:75:29)
at exports.printComments (./node_modules/recast/lib/comments.js:325:22)
at printWithComments (./node_modules/recast/lib/printer.js:61:16)
at FastPath.call (./node_modules/recast/lib/fast-path.js:119:16)
at genericPrintNoParens (./node_modules/recast/lib/printer.js:573:33)
at genericPrint (./node_modules/recast/lib/printer.js:164:9)
```

If I don't modify the comment, things are fine since it doesn't try to reprint it. If the semi-colon is placed before the comment, it will also work.

Start with the supplied JavaScript reproduction and trace the assertion from lib/patcher.js through lib/printer.js and lib/comments.js. Confirm the trailing comment case where the semicolon follows the identifier, then verify that printing the modified comment completes without an assertion while the existing semicolon-before-comment case still works.

Written by the indexing model from the issue text.