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

[Alex Bradbury via llvm-commits]

asb wrote:

I think I'm ok to merge with Philip's LGTM and I believe all outstanding comments have been addressed, but I'll give this another day or so as it's been open so long. Please speak up if there's anything you feel wasn't fully reflected in the current code.

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