[InstCombine] Add droppable users back to worklist (NFCI)

When sinking and users are dropped, add the using instructions
to the worklist, as they can likely be removed as well.

This should be NFC apart from worklist order effects.

llvm/lib/Transforms/InstCombine/InstCombineInternal.h

@@ -653,6 +653,8 @@ class LLVM_LIBRARY_VISIBILITY InstCombinerImpl final
   Instruction *SimplifyAnyMemSet(AnyMemSetInst *MI);
 
   Value *EvaluateInDifferentType(Value *V, Type *Ty, bool isSigned);
+
+  bool tryToSinkInstruction(Instruction *I, BasicBlock *DestBlock);
 };
 
 class Negator final {

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -3475,8 +3475,8 @@ static bool SoleWriteToDeadLocal(Instruction *I, TargetLibraryInfo &TLI) {
 /// beginning of DestBlock, which can only happen if it's safe to move the
 /// instruction past all of the instructions between it and the end of its
 /// block.
-static bool TryToSinkInstruction(Instruction *I, BasicBlock *DestBlock,
-                                 TargetLibraryInfo &TLI) {
+bool InstCombinerImpl::tryToSinkInstruction(Instruction *I,
+                                            BasicBlock *DestBlock) {
   BasicBlock *SrcBlock = I->getParent();
 
   // Cannot move control-flow-involving, volatile loads, vaarg, etc.
@@ -3523,10 +3523,13 @@ static bool TryToSinkInstruction(Instruction *I, BasicBlock *DestBlock,
         return false;
   }
 
-  I->dropDroppableUses([DestBlock](const Use *U) {
-    if (auto *I = dyn_cast<Instruction>(U->getUser()))
-      return I->getParent() != DestBlock;
-    return true;
+  I->dropDroppableUses([&](const Use *U) {
+    auto *I = dyn_cast<Instruction>(U->getUser());
+    if (I && I->getParent() != DestBlock) {
+      Worklist.add(I);
+      return true;
+    }
+    return false;
   });
   /// FIXME: We could remove droppable uses that are not dominated by
   /// the new position.
@@ -3699,7 +3702,7 @@ bool InstCombinerImpl::run() {
     if (OptBB) {
       auto *UserParent = *OptBB;
       // Okay, the CFG is simple enough, try to sink this instruction.
-      if (TryToSinkInstruction(I, UserParent, TLI)) {
+      if (tryToSinkInstruction(I, UserParent)) {
         LLVM_DEBUG(dbgs() << "IC: Sink: " << *I << '\n');
         MadeIRChange = true;
         // We'll add uses of the sunk instruction below, but since