[SLP]Improve comparison of shuffled loads/masked gathers by adding GEP cost.

In some cases masked gather is less profitable than insert-subvector of
consecutive/strided stores. SLP has this kind of analysis, but need to
improve it by adding the cost of the GEP analysis.
Also, the GEP cost estimation for masked gather is fixed.

Reviewers: RKSimon

Reviewed By: RKSimon

Pull Request: https://github.com/llvm/llvm-project/pull/90737