[llvm] [RISCV] Use simm8 instead of uimm8 for pli.b (PR #153913)

[Sam Elliott via llvm-commits]

lenary wrote:

We could also end up with a terrible "fix immediate" hack, but I'm not sure. I just worry about people writing `0xff` because that's clearer for their usecase, and it being rejected.

https://github.com/llvm/llvm-project/pull/153913