[clang] [llvm] [BPF] Add load-acquire and store-release instructions under -mcpu=v4 (PR #108636)

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4ast wrote:

> BPF_ATOMIC | BPF_DW | BPF_LDX

I'd rather not. See my earlier comment. Existing *fetch* flavors return the value.

https://github.com/llvm/llvm-project/pull/108636