[clang] [llvm] [RISCV] Add Smrnmi extension (PR #111668)

[Sam Elliott via cfe-commits]

================
@@ -813,6 +815,12 @@ def MRET : Priv<"mret", 0b0011000>, Sched<[]> {
   let rs1 = 0;
   let rs2 = 0b00010;
 }
+
+def MNRET : Priv<"mnret", 0b0111000>, Sched<[]> {
----------------
lenary wrote:

Does this need a `Requires=[HasStdExtSmrnmi]` (And a definition of `HasStdExtSmrnmi`)?

https://github.com/llvm/llvm-project/pull/111668