[llvm] [DirectX] add support for i64 buffer load/stores (PR #145047)

[Justin Bogner via llvm-commits]

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

A follow up to one of my previous suggestions and a few notes on misleading comments, but once those are taken care of this looks good.

https://github.com/llvm/llvm-project/pull/145047