[llvm] [IR] Initial introduction of memset_pattern (PR #97583)

[Nikita Popov via llvm-commits]

nikic wrote:

> Theoretically, in the long run, it would be nice if "memset" just was a subset of "memset.pattern" whit the pattern size matching the byte size. At a glance it seems like memset.pattern might fill the gap, but then all optimizations should deal with byte sized value argument in memset.pattern just as well as ordinary memset. I doubt that it will happen in the near future (or ever).

An alternative we should probably consider here is to not have a separate memset.pattern at all, and instead have memset be memset.pattern. If we're going to add a type overload to memset anyway for non-8-bit targets, there's probably not a lot of point to having a separate memset.pattern?

https://github.com/llvm/llvm-project/pull/97583