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

[Sam Elliott via llvm-commits]

lenary wrote:

To be clear, this pass is doing the right thing and I'm happy with it. The comments about what else we might expect are an aside, and I'm happy with Craig's explanation.

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