[llvm] [DirectX] Implement `memcpy` in DXIL CBuffer Access pass (PR #144436)

[Justin Bogner via llvm-commits]

https://github.com/bogner approved this pull request.

Looks good with a couple of minor comments. I expect all of this to be greatly simplified once my explicit padding for cbuffers proposal is ready, but this looks reasonable in the meantime.

https://github.com/llvm/llvm-project/pull/144436