[llvm] [PreISelIntrinsicLowering] Produce a memset_pattern16 libcall for llvm.experimental.memset.pattern when available (PR #120420)

[Philip Reames via llvm-commits]

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

LGTM w/minor comments addressed.  I think all of @arsenm's comments are addressed, so up to you if you want to wait for him to confirm.

https://github.com/llvm/llvm-project/pull/120420