[llvm] [RISCV] Select atomic_{load/store} to pseudos and expand them later (PR #67108)

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

If we want to make `fence` and `load/store` schedulable separately, we can just expand them before RA in `RISCVPreRAExpandPseudo`.

https://github.com/llvm/llvm-project/pull/67108