Usually IF is covered by the separate rule for heat calculation at long range, where only long range weapons' heat is counted. But the Masakari builds up so much heat at long range it's long range exclusive heat is still lowering the damage values, thus the OVL.
Overall, it was not something that was generally a problem, so there usually wasn't a need to another conversion rule to make an exception for IF. Because it wouldn't be a simple no heat calculation for IF, as then a unit could pile on LRMs for indirect fire without caring about heat at all. So it would require yet another entirely separate heat calculation rule...
Long story short, if you got a Masakari Prime for IF, you're doing it wrong anyway :).