[llvm] [NVPTX] Add syncscope support for cmpxchg (PR #140812)

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zhaorv wrote:

> Can we do this without modifying the base API? Looks like the SSID information is enclosed in the instruction which we can query in NVPTX specific `NVPTXTargetLowering::emitTrailingFence`/`NVPTXTargetLowering::emitLeadingFence`.

For what it's worth, I think it's helpful to modify the base API because some targets I have to support would also benefit from scoping fences emitted in `emitTrailingFence` and `emitLeadingFence`. Perhaps it could be done in a separate patch though.

https://github.com/llvm/llvm-project/pull/140812