Automerge: [SLP][NFC] Redesign schedule bundle, separate from schedule data, NFC

That's the initial patch, intended to support revectorization of the
previously vectorized scalars. If the scalar is marked for the
vectorization, it becomes a part of the schedule bundle, used to check
dependencies and then schedule tree entry scalars into a single batch of
instructions. Unfortunately, currently this info is part of the
ScheduleData struct and it does not allow making scalars part of many
bundles. The patch separates schedule bundles from the ScheduleData,
introduces explicit class ScheduleBundle for bundles, allowing later to
extend it to support revectorization of the previously vectorized
scalars.

Reviewers: hiraditya, RKSimon

Reviewed By: RKSimon, hiraditya

Pull Request: llvm/llvm-project#131625

Author: alexey-bataev