[llvm] [RISCV] Lower the paadd/pasub intrinsics to existing ISD nodes. NFC (PR #203646)

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sihuan wrote:

That makes sense, thanks — a fragile IR pattern isn't a guarantee we can hand intrinsic users, so keeping the explicit intrinsics is the right tradeoff. LGTM.

https://github.com/llvm/llvm-project/pull/203646