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

[Jessica Clarke via cfe-commits]

jrtc27 wrote:

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

Flushing the instruction cache doesn't make sense given it can never be dirty. Invalidating/discarding does, but that's just what fence.i is doing?

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