[llvm] [AArch64][LV] Reduce cost of scaled reduction extends (PR #134074)

[David Sherwood via llvm-commits]

================
@@ -1693,6 +1693,15 @@ InstructionCost VPWidenRecipe::computeCost(ElementCount VF,
     if (RHS->isLiveIn())
       RHSInfo = Ctx.TTI.getOperandInfo(RHS->getLiveInIRValue());
 
+    // The mul is folded into another target instruction when participating
+    // in scaled reductions.
+    if (Opcode == Instruction::Mul && !hasMoreThanOneUniqueUser()) {
+      if (all_of(users(), [](const VPUser *U) {
----------------
david-arm wrote:

Hmm, if there is only one unique user then we don't actually need to walk over every user, right? Also `hasMoreThanOneUniqueUser` can return false if there are no users at all. I wonder if it's better to add a new `getSingleUniqueUser` interface that does something like:

```
  VPUser *getSingleUniqueUser() const {
    if (getNumUsers() == 0)
      return nullptr;

    // Check if all users match the first user.
    auto Current = user_begin();
    while (Current != user_end()) {
      if (*user_begin() != *Current)
        return nullptr;
      Current++;
    }
    return *Current;
  }
```

then you could do something like:

```
  if (Opcode == Instruction::Mul) {
    auto *SingleUser = getSingleUniqueUser();
    if (SingleUser && isa_and_present<VPPartialReductionRecipe>(SingleUser))
      return TTI::TCC_Free;
  }

https://github.com/llvm/llvm-project/pull/134074