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

[Shao-Ce SUN via cfe-commits]

sunshaoce wrote:

By the way, is there any plan to support `CFLUSH.I.L1` in the future?

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