[clang] [llvm] [RISCV] Add back SiFive's cdiscard.d.l1, cflush.d.l1, and cease instructions. (PR #83896)

[Garvit Gupta via cfe-commits]

quic-garvgupt wrote:

I believe the patch is ready to be merged now, but since I am not the reviewer here, someone else must approve it as well.

Thanks again for the patch!

https://github.com/llvm/llvm-project/pull/83896