Probably nothing.

 

Since the encounter would take place in Iraqi airspace the US aircraft would need Iraqi permission to fire.  In addition protocols would require an attempt to force the Iranian aircraft down first, where they would just bribe the Iraqi’s to let them take off against after the nasty American aircraft have gone away.

 

And since the aircraft will have to abort a bombing mission (the USA does not have permission to run CAP) to intercept those aircraft the downside may not be acceptable to planners.  May have already happened and ignoring it could be unofficial policy to avoid an embarrassing situation like when Osama bin Laden was killed.