[llvm] [llvm][RISCV] Support P Extension CodeGen (PR #190303)

[Kito Cheng via llvm-commits]

https://github.com/kito-cheng commented:

The title is seems too generally, and maybe rename to "[RISCV] Select add(vec, splat(scalar)) to PADD_*S for P extension"?

https://github.com/llvm/llvm-project/pull/190303