[NFC][LazyCallGraph] Remove check in removeDeadFunction() if graph is empty

[aeubanks]

Summary

If we're in removeDeadFunction(), we should have already constructed the call graph.

[aeubanks]

ping

llvm/lib/Analysis/LazyCallGraph.cpp

if (NI == NodeMap.end())
     return;
 
   Node &N = *NI->second;
   NodeMap.erase(NI);
 
   // Remove this from the entry edges if present.
   EntryEdges.removeEdgeInternal(N);
 
-  if (SCCMap.empty()) {
-    // No SCCs have been formed, so removing this is fine and there is nothing
-    // else necessary at this point but clearing out the node.
-    N.clear();
-    return;
-  }
-
   // Cannot remove a function which has yet to be visited in the DFS walk, so
   // if we have a node at all then we must have an SCC and RefSCC.
   auto CI = SCCMap.find(&N);
   assert(CI != SCCMap.end() &&
          "Tried to remove a node without an SCC after DFS walk started!");
   SCC &C = *CI->second;
   SCCMap.erase(CI);
   RefSCC &RC = C.getOuterRefSCC();