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

[Garvit Gupta via cfe-commits]

quic-garvgupt wrote:

Thanks for the prompt reply and latest patchset.

1. Do we need to add documentation in RISCVUsage.rst file for xsfcease?
2. Also, as we are adding cease instruction in this PR, can we rename the PR to include the cease instruction as well?

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