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

[Alex Bradbury via llvm-commits]

asb wrote:

Re-requested review from @arsenm - I believe all review comments are addressed either with code modifications or responses to the comment (I think [this](https://github.com/llvm/llvm-project/pull/120420#discussion_r1916278793) is the main one to check).

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