[SLP][NFC]Unify ScalarToTreeEntries and MultiNodeScalars, NFC

Currently, SLP has 2 distinct storages to manage mapping between
vectorized instructions and their corresponding vectorized TreeEntry
nodes. It leads to inefficient lookup for the matching TreeEntries and
makes it harder to correctly track instructions, associated with
multiple nodes.
There is a plan to extend this support for instructions, that require
scheduling, to allow support for copyable elements. Merging
ScalarToTreeEntry and MultiNodeScalars will allow reduce maintenance of
the feature

Reviewers: RKSimon

Reviewed By: RKSimon

Pull Request: llvm#124914

Author: alexey-bataev