[llvm] [X86] Use fence(seq_cst) in IdempotentRMWIntoFencedLoad (PR #126521)

[Phoebe Wang via llvm-commits]

https://github.com/phoebewang approved this pull request.

I'm not an expert of atomic, but the changes in test and the old comments indicate it's correcrt to do so.

I'll give my +1 but it'd better if someone familar with atomic take another look.

https://github.com/llvm/llvm-project/pull/126521