[clang] [llvm] [RISCV][MC] Add assembler support for XRivosVisni (PR #128773)

[Sam Elliott via cfe-commits]

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@@ -25,3 +65,27 @@ defm RI_VZIP2B_V : VALU_IV_V<"ri.vzip2b", 0b010100>;
 defm RI_VUNZIP2A_V : VALU_IV_V<"ri.vunzip2a", 0b001000>;
 defm RI_VUNZIP2B_V : VALU_IV_V<"ri.vunzip2b", 0b011000>;
 }
+
+//===----------------------------------------------------------------------===//
+// XRivosVisni
+//===----------------------------------------------------------------------===//
+
+let Predicates = [HasVendorXRivosVisni], DecoderNamespace = "XRivos",
+  Inst<6-0> = OPC_CUSTOM_2.Value, mayLoad = false, mayStore = false,
+  hasSideEffects = false in {
+
+let isReMaterializable = 1, isAsCheapAsAMove = 1, vm = 0, vs2=0 in
+def RI_VZERO : RVInstV<0b000000, 0b00000, OPCFG, (outs VR:$vd),
+                     (ins), "ri.vzero", "$vd">;
----------------
lenary wrote:

```suggestion
                     (ins), "ri.vzero.v", "$vd">;
```
The document uses this mnemonic - should we use it here? I'm not entirely up to date on the V-extensions, and how mnemonics are generally handled there though, and what the conventions are.

https://github.com/llvm/llvm-project/pull/128773