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

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

> If we have Zalasr, wouldn't we disable the fence emission rather than trying to pattern match it?
> Yes, just don't expand them into memory access + fence... you don't need a pseudo for that

Yes, I agree. We can just do the instruction selection like AArch64, and the patterns will be like these in this PR expect that the selected instructions are `Zalasr` instructions instead of pseudos.

The existence of pseudos is to unify the implementation of `Zalasr` and fused atomic load/store, can this be a strong reason? And there is one more information that `MacroFusion` way may not be perfect because some fence+load/store are still seperated when postRA scheduler is enabled. After some rough investigation, this may be bacause there is no data dependency between fence and load/store and the dependency analysis is not so smart.


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