[CGSCC][NewPM] Fix adding mutually recursive new functions

When adding a new function via addNewFunctionIntoRefSCC(), it creates a
new node and immediately populates the edges. Since populateSlow() calls
G->get() on all referenced functions, it will create a node (but not
populate it) for functions that haven't yet been added. If we add two
mutually recursive functions, the assert that the node should never have
been created will fire when the second function is added. So here we
remove that assert since the node may have already been created (but not
yet populated).

createNode() is only called from addNewFunctionInto{,Ref}SCC().

https://bugs.llvm.org/show_bug.cgi?id=47502

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D87623