[llvm] [SLP][REVEC] SubVectorsMask should be transformed into vector form. (PR #120398)

[Alexey Bataev via llvm-commits]

https://github.com/alexey-bataev commented:

This whole code is currently not correct, need to match same functionality in CostEstimator. Also, `BaseShuffleAnalysis::getVF()` should return actual vectorization factor, not the one adjusted to 'scalar' element. Otherwise, at least cost is not estimated correctly

https://github.com/llvm/llvm-project/pull/120398