[Attributor] Introduce an API to delete stuff

[jdoerfert]

Summary

During the fixpoint iteration, including the manifest stage, we should
not delete stuff as other abstract attributes might have a reference to
the value. Through the API this can now be done safely at the very end.

[sstefan1]

LGTM.

llvm/include/llvm/Transforms/IPO/Attributor.h

struct Attributor {
   /// Note that abstract attribute instances are generally created even if the
   /// IR already contains the information they would deduce. The most important
   /// reason for this is the single interface, the one of the abstract attribute
   /// instance, which can be queried without the need to look at the IR in
   /// various places.
   void identifyDefaultAbstractAttributes(
       Function &F, DenseSet<const char *> *Whitelist = nullptr);
 
+  /// Record that \p I is deleted after information was manifested.
+  void deleteAfterManifest(Instruction &I) { ToBeDeletedInsts.insert(&I); }
+
+  /// Record that \p BB is deleted after information was manifested.
+  void deleteAfterManifest(BasicBlock &BB) { ToBeDeletedBlocks.insert(&BB); }
+
+  /// Record that \p F is deleted after information was manifested.
+  void deleteAfterManifest(Function &F) { ToBeDeletedFunctions.insert(&F); }
+
   /// Return true if \p AA (or its context instruction) is assumed dead.
   ///
   /// If \p LivenessAA is not provided it is queried.
   bool isAssumedDead(const AbstractAttribute &AA, const AAIsDead *LivenessAA);
 
   /// Check \p Pred on all function call sites.
   ///
   /// This method will evaluate \p Pred on call sites and return
private:
   ///{
   using QueryMapTy =
       MapVector<AbstractAttribute *, SetVector<AbstractAttribute *>>;
   QueryMapTy QueryMap;
   ///}
 
   /// The information cache that holds pre-processed (LLVM-IR) information.
   InformationCache &InfoCache;
+
+  /// Functions, blocks, and instructions we delete after manifest is done.
+  ///
+  ///{
+  SmallPtrSet<Function *, 8> ToBeDeletedFunctions;
+  SmallPtrSet<BasicBlock *, 8> ToBeDeletedBlocks;
+  SmallPtrSet<Instruction *, 8> ToBeDeletedInsts;
+  ///}
 };
 
 /// An interface to query the internal state of an abstract attribute.
 ///
 /// The abstract state is a minimal interface that allows the Attributor to
 /// communicate with the abstract attributes about their internal state without
 /// enforcing or exposing implementation details, e.g., the (existence of an)
 /// underlying lattice.

llvm/lib/Transforms/IPO/Attributor.cpp

ChangeStatus Attributor::run() {
 
   NumAttributesManifested += NumManifested;
   NumAttributesValidFixpoint += NumAtFixpoint;
 
   (void)NumFinalAAs;
   assert(
       NumFinalAAs == AllAbstractAttributes.size() &&
       "Expected the final number of abstract attributes to remain unchanged!");
+
+  // Delete stuff at the end to avoid invalid references and a nice order.
+  LLVM_DEBUG(dbgs() << "\n[Attributor] Delete " << ToBeDeletedFunctions.size()
+                    << " functions and " << ToBeDeletedBlocks.size()
+                    << " blocks and " << ToBeDeletedInsts.size()
+                    << " instructions\n");
+  for (Instruction *I : ToBeDeletedInsts) {
+    if (I->hasNUsesOrMore(1))
+      I->replaceAllUsesWith(UndefValue::get(I->getType()));
+    I->eraseFromParent();
+  }
+  for (BasicBlock *BB : ToBeDeletedBlocks) {
+    // TODO: Check if we need to replace users (PHIs, indirect branches?)
+    BB->eraseFromParent();
+  }
+  for (Function *Fn : ToBeDeletedFunctions) {
+    Fn->replaceAllUsesWith(UndefValue::get(Fn->getType()));
+    Fn->eraseFromParent();
+  }
+
   return ManifestChange;
 }
 
 /// Helper function that checks if an abstract attribute of type \p AAType
 /// should be created for IR position \p IRP and if so creates and registers it
 /// with the Attributor \p A.
 ///
 /// This method will look at the provided whitelist. If one is given and the