[llvm] [SLP] Pessimistically handle unknown vector entries in SLP vectorizer (PR #75438)

[Alexey Bataev via llvm-commits]

https://github.com/alexey-bataev approved this pull request.

Ok, let's fix it this way for now, later need to teach getInsertIndex to separate representation for undef/runtime indices

https://github.com/llvm/llvm-project/pull/75438