[Alignment][NFC] Migrate AtomicExpandPass to Align

This is a followup on D78403.
I'm unsure about `getAtomicOpAlign` overloads that take `AtomicRMWInst` and `AtomicCmpXchgInst`, shouldn't `getAlign` provide the correct answer already?

Differential Revision: https://reviews.llvm.org/D81369