[SLP]Initial support for copyable elements (non-schedulable only)

Adds initial support for copyable elements. This patch only models adds
and model copyable elements as add <element>, 0, i.e. uses identity
constants for missing lanes.
Only support for elements, which do not require scheduling, is added to
reduce size of the patch.

Reviewers: RKSimon, hiraditya

Reviewed By: RKSimon

Pull Request: llvm#140279

Author: alexey-bataev