[ctx_prof] PGOCtxProfFlattener must always return PreservedAnalyses::none()

This is because it always removes instrumentation. This fixes failures
detectable with extensive checks, e.g. https://lab.llvm.org/buildbot/#/builders/187/builds/987

(Related to PR #107329)

Author: mtrofin

llvm/lib/Transforms/Instrumentation/PGOCtxProfFlattening.cpp

@@ -315,7 +315,7 @@ PreservedAnalyses PGOCtxProfFlatteningPass::run(Module &M,
   });
   auto &CtxProf = MAM.getResult<CtxProfAnalysis>(M);
   if (!CtxProf)
-    return PreservedAnalyses::all();
+    return PreservedAnalyses::none();
 
   const auto FlattenedProfile = CtxProf.flatten();