[IR] simplify code in removePredecessor(); NFCI

As suggested in D92247 (and independent of whatever we decide to do there),
this code is confusing as-is. Hopefully, this is at least mildly better.

We might be able to do better still, but we have a function called
"removePredecessor" with this behavior:
"Note that this function does not actually remove the predecessor." (!)