[clang] [llvm] [AMDGPU] Add an option to disable unsafe uses of atomic xor (PR #69229)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> @arsenm That makes sense, I don't think MMRA fits the fine-grained use case either. Does that mean we can stick with the approach from this PR? @b-sumner mentioned there was another similar approach being worked on.

Something like this, but the naming and direction of this PR is backwards. The default should be assume fine grained memory is possible. We also have another orthogonal bit we need to track in addition to fine grained memory. I was envisioning this as a single integer interpreted as bitfields of the two, but this uses different key:value metadata pairs. It should be named "amdgpu.no.something" to assert the operation doesn't access fine grained 

https://github.com/llvm/llvm-project/pull/69229