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

[Alex Bradbury via llvm-commits]

asb wrote:

Just a holding pattern update to explain the last push:
I've merged in the latest changes and adjusted to match the upstream. I've pushed so we can work from a relatively up to date base.
* I know my changes slightly undo some of the refactoring Philip did. I will revisit.
* I do still see some unwanted/unexpected changes and am looking to understand those at the moment.

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