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: https://github.com/llvm/llvm-project/pull/140279