planner: fix ref heads processing (#5418)

With the introduction of ref heads in #4660, the planned IR
still mostly worked, but it was bypassing the CallDynamic
optimization when it shouldn't have.

This commit re-works some of the rule planning to more robustly
handle ref heads.

Also adds a few test cases to get a grip on what should and
should not happen.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>