[RISCV] Split the pseudo instruction splitting pass

Extracts the atomic pseudo-instructions' splitting from `riscv-expand-pseudo`
/ `RISCVExpandPseudo` into its own pass, `riscv-expand-atomic-pseudo` /
`RISCVExpandAtomicPseudo`. This allows for the expansion of atomic operations
to continue to happen late (the new pass is added in `addPreEmitPass2`, so
those expansions continue to happen in the same place), while the remaining
pseudo-instructions can now be expanded earlier and benefit from more
optimization passes. The nonatomics pass is now added in `addPreSched2`.

Differential Revision: https://reviews.llvm.org/D79635