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

[Alex Bradbury via llvm-commits]

asb wrote:

> I've merged in the latest changes and will re-check my comparison.

Confirming that no codegen changes stand out to me - there are some minor diffs, but this is vastly reduced vs when this started and most seem to be from slightly different loop unrolling behaviour.

I'm going to fix up the commit message and merge now, as this has had positive review and I believe comments are all addressed.

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