[llvm] [DirectX] Mark buffer load/store as mem read/write (PR #114105)

[Helena Kotas via llvm-commits]

hekota wrote:

This is certainly a step in the right direction, but it will probably not eliminate extra `load`a on a handle after a `typedBufferStore` is used on the same buffer, right? Or is that part of the "broader audit" you mentioned above?

https://github.com/llvm/llvm-project/pull/114105