[SLP]Add GEP cost estimation for gathered loads.

When doing estimation for vectorization of gathered loads, need to
estimate the cost of the pointer (vectorization), as it is done for the
actual vectorized loads. Otherwise may be too optimistic about the cost
of the gathered loads.

Reviewers: preames

Reviewed By: preames

Pull Request: https://github.com/llvm/llvm-project/pull/80867