[Coroutines][LazyCallGraph] resumes are not really SCC

In this debug code the check assumes the lookup find an object. If it
does not it tries to dereference the nullptr. This patch adds a check,
however, notice it does not require the object to exist.

This method (addSplitRefRecursiveFunctions) was added for coroutines
with continuations. The split coroutine transform generates the
continuations by cloning the entire function then changing the entry
block and a few other things. This results in a lot of dead code that is
kept around only to satisfy a few assumptions then is removed.

With the deadcode the original function and its continuations appear to
be SCC. However, when the deadcode is removed they are not SCC. Consider
a simple coroutine { begin ... suspend ... suspend ... end }. After
deadcode is eliminated the ramp references resume0, resume0 references
resume1, etc...

To be efficient it is necessary to split coroutines without generating
deadcode and this removese the unnecessary references (on a phi's edges).
This does not satisfy one of the conditions of addSplitRefRecursiveFunctions:
"All new functions must reference (not call) each other.".

From my testing so far it seems that this condition is not necessary and
it is only this debug code that checks for it.

Can we safely remove the "All new functions must reference (not call) each
other." requirement of addSplitRefRecursiveFunctions?

If not, what alternative do we have so we avoid cloning deadcode? (In
our use case the deadcode can be a particular problem due to its size).

Author: tnowicki

llvm/lib/Analysis/LazyCallGraph.cpp

@@ -1775,8 +1775,9 @@ void LazyCallGraph::addSplitRefRecursiveFunctions(
       if (F1 == F2)
         continue;
       Node &N2 = get(*F2);
-      assert(!N1->lookup(N2)->isCall() &&
-             "Edges between new functions must be ref edges");
+      assert(!N1->lookup(N2) ||
+             (!N1->lookup(N2)->isCall() &&
+              "Edges between new functions must be ref edges"));
     }
   }
 #endif