[SLP]Introduce transformNodes() and transform loads + reverse to strided loads.

Introduced transformNodes() function to perform transformation of the
nodes (cost-based, instruction count based, etc.).
Implemented transformation of consecutive loads + reverse order to
strided loads with stride -1, if profitable.

Reviewers: RKSimon, preames, topperc

Reviewed By: RKSimon

Pull Request: https://github.com/llvm/llvm-project/pull/88530