[llvm] [LV] Add test showing lack of gather/scatter can prevent if-convert (PR #195042)

[Gaëtan Bossu via llvm-commits]

gbossu wrote:

Discussed offline: I'll split that patch further into two others:
- introduce `isLegalMaskedLoadOrStore` as a purely NFC change and use it only in `isScalarWithPredication`
- Use the new `isLegalMaskedLoadOrStore` for interleaved accesses as well

https://github.com/llvm/llvm-project/pull/195042