[llvm] [LV] Remove legacy selectVectorizationFactor and assert (NFCI) (PR #190838)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM. This will unblock a lot of TODOs and potential simplifications in VPlanRecipes.cpp 

@Mel-Chen I think after this we can now cost reversed masks in loads/stores now.

And off the top of my head isUsedByLoadStoreAddress can probably have some of the logic around handling blends simplified.

https://github.com/llvm/llvm-project/pull/190838