[SimplifyCFG] Don't split predecessors of callbr terminator

This addresses the assertion failure reported in
https://reviews.llvm.org/D124159#3631240.

I believe that this limitation in SplitBlockPredecessors is not
actually necessary (because unlike with indirectbr, callbr is
restricted in a way that does allow updating successors), but for
now fix the assertion failure the same way we do everywhere else,
by also skipping callbr.