Ok, here’s the X’ structure for “to never split infinitives” (simplified, and quite dependent on how much of my syntax class I remember):
I'__
/ \
Iº _VP___
| / \_
to AdvP V'_
/_\ | \
never Vº NP
| /_\
split infinitives
“never split infinitives” is a verb phrase; “split infinitives” is a verb sub-phrase, and “split” is the head of the verb phrase. “To” is the head of the inflectional subphrase (never mind), whose structure is [Iº VP]. So it’s fine to put the head of the inflectional phrase (“to”) before the verb phrase (“never split infinitives”).
(In fact, I’m having a little difficulty figuring out how to draw a tree for the supposedly more proper “never to split infinitives”, so I won’t bother. Also, drawing syntax trees in this is a pain in the ass.)