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

[Han-Kuan Chen via llvm-commits]

HanKuanChen wrote:

> > `ShuffleCostEstimator::createShuffle` can handle `FixedVectorType` now. It expands `CommonMask` into a correct mask before calling `BaseShuffleAnalysis::createShuffle`. We just need to make `GetNodeMinBWAffectedCost` and `GetValueMinBWAffectedCost` support `FixedVectorType`.
> > `ShuffleInstructionBuilder::finalize` can handle `FixedVectorType` now. Any development that modifies the mask can be easily identified and may cause a crash. At that point, we can quickly fix the bug.
> > I think the real solution is to wrap the mask into a class. This class can distinguish whether we are vectorizing a scalar to a vector or a vector to a vector. Additionally, we can add a cost model and an instruction builder within the class.
> 
> Bad decision. As I said, these classes should not know anything about high level abstraction, they should operate on actual masks/data

I think we have some misunderstanding here. They already operate the actual mask.

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