[llvm] [RISCV] Port ExpandAtomicPseudo to NewPM (PR #218161)

[Sam Elliott via llvm-commits]

lenary wrote:

I think n=4 is enough duplication that I'm going to work on reducing it a little, and trying to get all the pass names to line up a bit better.

https://github.com/llvm/llvm-project/pull/218161