[SLP]Improve/fix extracts calculations for non-power-of-2 elements.

One of the previous patches introduced initial support for non-power-of-2
number of elements but some parts of the SLP vectorizer still were not
adjusted to handle the costs correctly. Patch fixes it by improving
analysis of the non-power-of-2 number of elements and fixes in the cost
of the extractelements instructions.

Reviewers: RKSimon

Reviewed By: RKSimon

Pull Request: https://github.com/llvm/llvm-project/pull/93213