[llvm] [loop-vectorize] Fix crash when using types that aren't known scale factors (PR #136680)

[Florian Hahn via llvm-commits]

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

LGTM, thanks.

Would be good to update the PR title `[loop-vectorize]` -> `[LV]` which is more commonly used to help people using common filters ;) 

https://github.com/llvm/llvm-project/pull/136680