[llvm] [SLP]Initial support for copyable elements (non-schedulable only) (PR #140279)

[Philip Reames via llvm-commits]

preames wrote:

> Adds initial support for copyable elements. This patch only models adds
and model copyable elements as add , 0.

Terminology wise, this would be much clearer if you described this as using identity constants for missing lanes.  The key bit in the above is the 0 is the identity value for add.  

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