[llvm] [VPlan] Explicitly reassociate header mask in logical and (PR #180898)

[Luke Lau via llvm-commits]

================
@@ -1614,6 +1605,31 @@ void VPlanTransforms::simplifyRecipes(VPlan &Plan) {
   }
 }
 
+/// Reassociate (headermask && x) && y -> headermask && (x && y) to allow the
+/// header mask to be simplified further, e.g. in optimizeEVLMasks.
+static void reassociateHeaderMask(VPlan &Plan) {
+  VPValue *HeaderMask = vputils::findHeaderMask(Plan);
+  if (!HeaderMask)
+    return;
+  SmallSetVector<VPUser *, 8> Worklist(HeaderMask->user_begin(),
+                                       HeaderMask->user_end());
+  while (!Worklist.empty()) {
----------------
lukel97 wrote:

By re-add do you mean a recipe is popped off the worklist and then added back? I think we only traverse users through logical-ands so there should never be a cycle, I don't think it can be added back. 

I tried iterating with an index but it runs into problems, it looks like insert_range can invalidate the iterator? i.e. this ends up crashing trying to dereference what I presume is the end of an old iterator.

```c++
  for (auto It = Worklist.begin(); It != Worklist.end(); ++It) {
    VPUser *U = *It;
    auto *R = dyn_cast<VPSingleDefRecipe>(U);
    if (!R)
      continue;
```

https://github.com/llvm/llvm-project/pull/180898