[R] Improve autosplice cpp code

[asfimport]

Followup to ARROW-5178. See discussion on #4704. 

Reporter: Neal Richardson / @nealrichardson

_{Note: This issue was originally created as ARROW-5761. Please see the migration documentation for further details.}

[thisisnic]

This issue hasn't had activity in a long time. If it's still being worked on, please leave a comment. Otherwise, it will be closed on 23rd June.

Labelled Status: Stale-Warning for tracking.