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

[Alex Bradbury via llvm-commits]

asb wrote:

> I think I agree with @nikic that long term, what we want here is not a separate memset.pattern, but a type parameter to memset. However, I'm leery of this being stuck in review indefinitely. Perfection is the enemy of the good.
> 
> Here's my suggestion on a path forward here:
> <snip>

Philip's plan seems sensible to me - just wanted to check if reviewers who have participated so far are (@nikic? @arsenm?). If so I'll go ahead and update along those lines. Thanks!

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