[llvm] [RISCV] Support P extension ABSW instruction. (PR #165047)

[Brandon Wu via llvm-commits]

4vtomat wrote:

> Where's the spec of the P extension? I don't see it at https://github.com/riscv/riscv-isa-manual and at riscv.org I found https://riscv.atlassian.net/browse/RVS-1135 which links to https://github.com/riscv/riscv-p-spec which contains `KABSW` not `ABSW`.

It's defined here: https://www.jhauser.us/RISCV/ext-P/RVP-baseInstrs-Sail-017.txt

https://github.com/llvm/llvm-project/pull/165047