[llvm] [AArch64][CodeGen] Add patterns for +lsfe atomic stores (PR #131174)

[Tomas Matheson via llvm-commits]

tmatheson-arm wrote:

For future reference, here is a proposal to add "atomic reduction operations" that would be able to make use of `STADD`/`STFADD` etc: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3111r1.html

I wonder whether we should add the tests anyway, to check that they _don't_ lower to `ST[F]ADD`?

https://github.com/llvm/llvm-project/pull/131174