[llvm] [SLP] Remove -slp-optimize-identity-hor-reduction-ops option (PR #106238)

[Philip Reames via llvm-commits]

preames wrote:

> This option was requested by Intel engs before, not sure they are ready to remove it.

In general, as a project, we actively do not support downstream forks with otherwise unneeded upstream code.  Unless there's a clear problem expressed, I'd still like to go ahead and remove the code.  They can apply a downstream patch if they want, or report an issue against the upstream.  

https://github.com/llvm/llvm-project/pull/106238