[AtomicExpandPass][NFC] Refactor processAtomicInstr to be more readable (#186547)
While working on
https://discourse.llvm.org/t/rfc-add-elementwise-modifier-to-atomicrmw/90134/5
I found this `processAtomicInstr` to be a little hard to read, with
casing on the instruction type all over the place. I think it reads
nicer to just case on the instruction type once.