SplitBlockPredecessors currently asserts if one of the predecessor terminators is a callbr. This limitation was originally necessary, because just like with indirectbr, it was not possible to replace successors of a callbr. However, this is no longer the case since D67252. As the requirement nowadays is that callbr must reference all blockaddrs directly in the call arguments, and these get automatically updated when setSuccessor() is called, we no longer need this limitation.
The only thing we need to do here is use replaceSuccessorWith() instead of replaceUsesOfWith(), because only the former does the necessary blockaddr updating magic.
I believe there's other similar limitations that can be removed, e.g. related to critical edge splitting.