[llvm] [LoopIdiom] Select llvm.experimental.memset.pattern intrinsic rather than memset_pattern16 libcall (PR #126736)

[Philip Reames via llvm-commits]

https://github.com/preames commented:

FYI, I reorganized the code a bit, so you'll have to rebase.  Feel free to defer until after answering my high level question though - I'm suspecting we can simplify this a bit.  

https://github.com/llvm/llvm-project/pull/126736