make ExecInsertIndexTuples arguments less bad

Hello,

The arguments to ExecInsertIndexTuples() are rather unhelpful to read;
patching them is messy and hard to follow.  How about we reuse the
pattern we used in commit f831d4accda0 to make them less bad?
I think the code is much nicer to read this way.

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"If you have nothing to say, maybe you need just the right tool to help you
not say it."                   (New York Times, about Microsoft PowerPoint)