[llvm] [SLP][REVEC] Fix CommonMask is transformed into vector form but used outside finalize. (PR #120952)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> Take the following code as an example.
> 
> ```
>     if (Action) {
>       Value *Vec = InVectors.front();
>       if (InVectors.size() == 2) {
>         Vec = createShuffle(Vec, InVectors.back(), CommonMask);
>         InVectors.pop_back();
>       } else {
>         Vec = createShuffle(Vec, nullptr, CommonMask);
>       }
>       for (unsigned Idx = 0, Sz = CommonMask.size(); Idx < Sz; ++Idx)
>         if (CommonMask[Idx] != PoisonMaskElem)
>           CommonMask[Idx] = Idx;
>       assert(VF > 0 &&
>              "Expected vector length for the final value before action.");
>       unsigned VecVF = cast<FixedVectorType>(Vec->getType())->getNumElements();
>       if (VecVF < VF) {
>         SmallVector<int> ResizeMask(VF, PoisonMaskElem);
>         std::iota(ResizeMask.begin(), std::next(ResizeMask.begin(), VecVF), 0);
>         Vec = createShuffle(Vec, nullptr, ResizeMask);
>       }
>       Action(Vec, CommonMask);
>       InVectors.front() = Vec;
>     }
> ```
> 
> How do you know `<4 x Ty>` from `unsigned VecVF = cast<FixedVectorType>(Vec->getType())->getNumElements();` is two `<2 x Ty>` (so VF is 2) instead of four `Ty` (so VF is 4)?

It should be considered as four `Ty`, we should operate on actual types here

https://github.com/llvm/llvm-project/pull/120952