[ctx_prof] Avoid llvm::append_range to fix some build bots

Example: https://lab.llvm.org/buildbot/#/builders/169/builds/3381

The CI allowed the `llvm::append_range` instantiation, but
on the other hand it's quite unnecessary here.

Author: mtrofin

llvm/lib/Transforms/Instrumentation/PGOCtxProfFlattening.cpp

@@ -246,7 +246,7 @@ class ProfileAnnotator final {
         continue;
       }
       if (succ_size(BB) == 1) {
-        llvm::append_range(Worklist, successors(BB));
+        Worklist.push_back(BB->getUniqueSuccessor());
         continue;
       }
       const auto &BBInfo = getBBInfo(*BB);