[IRBuilder] Add assert for AtomicRMW ordering

Add assert for AtomicRMW: Ordering != AtomicOrdering::Unordered
(https://github.com/llvm/llvm-project/blob/main/llvm/lib/IR/Verifier.cpp#L3944)
and adjust expandAtomicStore accordingly.

Test plan:
1/ ninja check-llvm check-clang check-lld
2/ Bootstrapped LLVM/Clang pass tests

Differential revision: https://reviews.llvm.org/D130457