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

[Craig Topper via cfe-commits]

topperc 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?

A cflush.i.l1 did appear in some SiFive manual at some point, but I don't think any hardware implements it.

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