Option 1: Forcing revisit of the SCC if there were any coroutine in it.

[GorNishanov]

Summary

What I don't like with this approach hard coding the information about coroutines in the general purpose facility.
We discussed during lunch another place for this check, namely RefreshSCC, where if we discover a suspend point, we will set Devirt to true.

Cost of doing it in CGPass::RunOnModule is linear with the number of functions.
Cost of doing it in CGPassManager::RefreshCallGraph: is linear with the number of instructions in the module. Hence out of this two, I weakly prefer the one shown in this Diff.

lib/Analysis/CallGraphSCCPass.cpp

bool CGPassManager::RunAllPassesOnSCC(CallGraphSCC &CurSCC, CallGraph &CG,
   
   // If the callgraph was left out of date (because the last pass run was a
   // functionpass), refresh it before we move on to the next SCC.
   if (!CallGraphUpToDate)
     DevirtualizedCall |= RefreshCallGraph(CurSCC, CG, false);
   return Changed;
 }
 
+static bool hasCoroutines(CallGraphSCC const& CurSCC) {
+  for (CallGraphNode *CGN : CurSCC)
+    if (auto F = CGN->getFunction())
+      if (F->hasFnAttribute(Attribute::Coroutine))
+        return true;
+  return false;
+}
+
 /// Execute all of the passes scheduled for execution.  Keep track of
 /// whether any of the passes modifies the module, and if so, return true.
 bool CGPassManager::runOnModule(Module &M) {
   CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
   bool Changed = doInitialization(CG);
   
   // Walk the callgraph in bottom-up SCC order.
   scc_iterator<CallGraph*> CGI = scc_begin(&CG);
while (!CGI.isAtEnd()) {
     // compile time in the case that we're making progress.  We also have a hard
     // iteration count limit in case there is crazy code.
     unsigned Iteration = 0;
     bool DevirtualizedCall = false;
     do {
       DEBUG(if (Iteration)
               dbgs() << "  SCCPASSMGR: Re-visiting SCC, iteration #"
                      << Iteration << '\n');
-      DevirtualizedCall = false;
+      DevirtualizedCall = hasCoroutines(CurSCC);
       Changed |= RunAllPassesOnSCC(CurSCC, CG, DevirtualizedCall);
     } while (Iteration++ < MaxIterations && DevirtualizedCall);
     
     if (DevirtualizedCall)
       DEBUG(dbgs() << "  CGSCCPASSMGR: Stopped iteration after " << Iteration
                    << " times, due to -max-cg-scc-iterations\n");
     
     if (Iteration > MaxSCCIterations)