[CGSCC] Verify that call graph is valid after iteration (llvm#94692)

Only in expensive checks, to match other LazyCallGraph verification.

Is helpful for verifying LazyCallGraph updates. Many issues only surface
when we reuse the LazyCallGraph.

(cherry picked from commit 6f2c610)

Author: aeubanks

llvm/include/llvm/Analysis/LazyCallGraph.h

@@ -943,6 +943,11 @@ class LazyCallGraph {
   LazyCallGraph(LazyCallGraph &&G);
   LazyCallGraph &operator=(LazyCallGraph &&RHS);
 
+#if !defined(NDEBUG) || defined(EXPENSIVE_CHECKS)
+  /// Verify that every RefSCC is valid.
+  void verify();
+#endif
+
   bool invalidate(Module &, const PreservedAnalyses &PA,
                   ModuleAnalysisManager::Invalidator &);
 

llvm/lib/Analysis/CGSCCPassManager.cpp

@@ -344,6 +344,11 @@ ModuleToPostOrderCGSCCPassAdaptor::run(Module &M, ModuleAnalysisManager &AM) {
     } while (!RCWorklist.empty());
   }
 
+#if defined(EXPENSIVE_CHECKS)
+  // Verify that the call graph is still valid.
+  CG.verify();
+#endif
+
   // By definition we preserve the call garph, all SCC analyses, and the
   // analysis proxies by handling them above and in any nested pass managers.
   PA.preserveSet<AllAnalysesOn<LazyCallGraph::SCC>>();

llvm/lib/Analysis/LazyCallGraph.cpp

@@ -211,6 +211,14 @@ LazyCallGraph::LazyCallGraph(LazyCallGraph &&G)
   updateGraphPtrs();
 }
 
+#if !defined(NDEBUG) || defined(EXPENSIVE_CHECKS)
+void LazyCallGraph::verify() {
+  for (RefSCC &RC : postorder_ref_sccs()) {
+    RC.verify();
+  }
+}
+#endif
+
 bool LazyCallGraph::invalidate(Module &, const PreservedAnalyses &PA,
                                ModuleAnalysisManager::Invalidator &) {
   // Check whether the analysis, all analyses on functions, or the function's