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

[Garvit Gupta via llvm-commits]

quic-garvgupt wrote:

Thanks for clearing the confusion around whether rs1 would be optional or not. Can we also add lit tests for the aliases? 

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