Fix infinite looping when loading a LLVM bitcode module that has llvm.coro.id.retcon.once intrinsics

rdar://149785911

Author: t-rasmud

llvm/lib/IR/AutoUpgrade.cpp

@@ -1139,10 +1139,13 @@ static bool upgradeIntrinsicFunction1(Function *F, Function *&NewFn,
     }
 
     if (F->arg_size() == 6 && Name == "coro.id.retcon.once") {
-      rename(F);
-      NewFn = Intrinsic::getDeclaration(F->getParent(),
-                                        Intrinsic::coro_id_retcon_once);
-      return true;
+      auto *FT = F->getFunctionType();
+      if (!FT->isVarArg()) {
+        rename(F);
+        NewFn = Intrinsic::getDeclaration(F->getParent(),
+                                          Intrinsic::coro_id_retcon_once);
+        return true;
+      }
     }
 
     break;