[clang] [llvm] [RISCV][MC] Add experimental Smidctrl, Ssidctrl, Smnip, and Ssnip support (PR #218108)

[Zeyi Xu via cfe-commits]

zeyi2 wrote:

Since this PR has been previously approved and I've addressed the open review feedback. I'd like to merge this later. If anything breaks, I'll do a quick revert :)

https://github.com/llvm/llvm-project/pull/218108