Follow up to
commit 2fe457690da0 ("Filter callbr insts from critical edge splitting")
@void and @efriedma pointed out in D88438 that we can split a critical
edge from a CallBrInst if that edge is to the default (non-indirect)
Permit this such an edge to be split for LICM, and as a result of
D88438, we can now drop the guard from llvm::SplitAllCriticalEdges() as
it will simply call into llvm::SplitCriticalEdge() and hit the guard
there that was newly added in D88438.
Realistically, we shouldn't often see a CallBrInst will no indirect
destinations (otherwise, why use CallBrInst, as CallInst would suffice),
but still a modest cleanup of llvm::SplitAllCriticalEdges().