I noticed $subject while chasing a different issue.
The way to fix this is to export pg_operator.c's makeOperatorDependencies function so that AlterOperator can call it. But since that declaration uses HeapTuple, I doubt we want to put it into pg_operator.h (which'd require further growth in that file's include list, and possibly break client-side code that wants to #include it). I'm inclined to split out pg_operator.h's function declarations into a new header pg_operator_fn.h, as we've already done in related cases.
While this isn't exactly a stop-ship class of problem, it still seems like it'd be good to fix before 9.5.0.