[llvm] [llvm][RISCV] Support mulh for P extension codegen (PR #171581)

[Brandon Wu via llvm-commits]

4vtomat wrote:

> For future reference, it's much easier to review if your PR contains two commits: one to add the tests, then a separate commit to implement the codegen and update the test outputs.

That make sense, I'll remind that, thanks!

https://github.com/llvm/llvm-project/pull/171581