[llvm] [IR] Add MemsetPatternInst to the MemIntrinsic hierarchy (PR #153301)

[Nikita Popov via llvm-commits]

nikic wrote:

This needs a rebase, but I think the change here is generally fine.

As for your last comment, I think long-term the best thing to do have the length in elements in the atomic intrinsics, but I don't think it needs to block this work. The MemIntrinsic hierarchy is much more important and widely used than the AnyMemIntrinsic hierarchy.

https://github.com/llvm/llvm-project/pull/153301