If-sentences, as well as all compound sentences, contain the principle clause and a subordinate one.
If the principle clause implies continuation, or each of the clauses is semantically independent they arrange separate intonation groups.
The principle clause may be pronounced with the falling, rising or even falling-rising intonation.
The subordinate clause usually implies non-finality and suggests not only continuation, but also sounds somewhat casual, e.g.:
If that’s how you want it, don’t ask me to help again.
If you ever need me, I’ll willingly help.