verifyFunction/verifyModule don't assert or error internally. They
also don't print anything if you don't pass a raw_ostream to them.
So the caller needs to check the result and ideally pass a stream
to get the messages. Otherwise they're just really expensive no-ops.
I've replaced calls inside LLVM_DEBUG with assert. The call in
CoroSplit should probably be in an assert, but I don't know
enough about the coroutine pipeline health.
Nit: I don't know what the convention is, &errs() might be better, but no strong feelings.