[SLP]Fix/improve potential masked gather loads analysis.

When do the analysis for the (potential) masked gather node, we check
that not greater than half of  the pointer operands are loop invariants
or potentially vectorizable.
Need to check actually, that we have a loop at first
and do better check for the potentially vectorizable
pointers.

Reviewers: RKSimon

Reviewed By: RKSimon

Pull Request: https://github.com/llvm/llvm-project/pull/83472