[llvm] [TTI] Provide a cost for memset_pattern which matches the libcall (PR #139978)

[Alex Bradbury via llvm-commits]

https://github.com/asb approved this pull request.

I think this is a pragmatic migration aid - makes sense to me. Perhaps the comment could be adjusted to make it clearer that this isn't necessarily the best cost long term. e.g. "This cost is set to match the cost of the memset_pattern16 libcall. It should likely be re-evaluated after migration to this intrinsic is complete."

https://github.com/llvm/llvm-project/pull/139978