[llvm] [LoopIdiom] Add minimal support for using llvm.experimental.memset.pattern (PR #118632)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

In terms of overall approach, I'd like to have memset.pattern -> memset_pattern16 libcall legalization support in the backend, and then have this patch unconditionally emit the intrinsic, without the option.

https://github.com/llvm/llvm-project/pull/118632