[llvm] [RISCV] Use PADD_DW instead of ADDD for GPRPair copy on RV32 with P extension (PR #181316)

[Craig Topper via llvm-commits]

topperc wrote:

> LGTM.
> 
> What was the reasoning for one vs the other? Is it just that `padd.dw` more clearly doesn't have a 64-bit carry?

Maybe. I asked but haven't got an answer yet.

https://github.com/llvm/llvm-project/pull/181316