[CoroSplit][NewPM] Don't call LazyCallGraph functions to split when no clones

Apparently there can be no clones, as happens in
coro-retcon-unreachable.ll.

The alternative is to allow no split functions in
addSplitRefRecursiveFunctions(), but it seems better to have the caller
make sure it's not accidentally splitting no functions out.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D94258