Placeholder isn't updated when nested tab stop changes

[dcampos]

When a mirror tab stop is placed within a placeholder, its value is correctly mirrored correctly from the tab stop it refers to, but its parent placeholder's bounds aren't updated to include the mirrored value.

Steps to reproduce

1. Define a Lua snippet like local ${1:module} = ${2:$1}.
2. Expand the snippet.
3. Change the first value to 'snippy'
4. Jump to the next placeholder.

Expected behavior

Neovim should be in select mode and the word 'snippy' should become selected.

Actual behavior

Cursor is placed after the the mirrored word 'snippy' in insert mode.