[RISCV] Temporarily move riscv-expand-pseudo pass to PreEmitPass2

The pass to split atomic and non-atomic RISC-V pseudo-instructions was itself
split into two passes in D79635 / commit rG2cb0644f, with the splitting of
non-atomic instructions being moved to the PreSched2 phase. A comment was
added to D79635 detailing a case where this caused problems, so this commit
moves the non-atomic split pass back to the PreEmitPass2 phase. This allows
the bulk of the changes from D79635 to remain committed, while addressing the
the reported problem (the pass split is now almost NFC). Once the root problem
is fixed we can move the (non-atomic) instruction splitting pass back to
earlier in the pipeline.