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

[Alex Bradbury via llvm-commits]

asb wrote:

I believe all review comments are addressed and this is ready for re-review. Thank you. Although nikic gave a LGTM, I'll await @arsenm's feedback as you had a lot of comments coming in after that.

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