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

[Alex Bradbury via llvm-commits]

asb wrote:

> Does this need to care about big endian vs little endian? I notice LoopIdiomRecognize.cpp only handles little endian targets.

You're right, I've adjusted the current implementation to bail out if targeting a big endian system.

With additional tests added and some cleanup now done, I think this is ready for review as is.

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