[llvm] [RISCV] Support codegen for some scalar P extension instructions. (PR #164359)

[Kito Cheng via llvm-commits]

https://github.com/kito-cheng approved this pull request.

Chat with Craig offline, and reviewed that, the only concern to me was the ABS instruction mixed in the PR, and now it removed, so LGTM

https://github.com/llvm/llvm-project/pull/164359