[llvm] [AArch64LoadStoreOpt] Allow monotonic atomics to be paired (PR #160580)

[Tomas Matheson via llvm-commits]

tmatheson-arm wrote:

For the LDP/STP cases, I can't see any particular reason this would be problematic. Pre-LSE2 these are treated as two single-copy 64 bit atomic ops, which is what we start with. Post-LSE2 they become one single-copy 128 bit atomic op, which is more strict. Mixing objects built with/without LSE2 and compiled before/after this patch should only ever introduce stricter ordering requirements, so there should be no ABI break introduced.

It would be good to see more tests showing that the other cases mentioned by @efriedma-quic not combined.

https://github.com/llvm/llvm-project/pull/160580