[llvm] [RISCV][P-ext] Merge paired pli/plui in RISCVMoveMerger (PR #202566)

[via llvm-commits]

sihuan wrote:

> > LGTM.
> > I guess i do find it interesting that we end up with `pli.b rN, imm; pli.b rN+1, imm` rather than e.g. `pli.b rN, imm; {c.}mv rN+1, rN`. I don't think it's a problem this isn't supported, if isel never actually creates that sequence though (but maybe it should?)
> 
> We have pli.b marked as rematerializable so that's probably why it doesn't do the mv.

Thanks for the context. Lines up with this pass: rematerialization gives 2x `pli.b` -> `pli.db`.
Planning to merge if there's nothing else.

https://github.com/llvm/llvm-project/pull/202566