nak: make Instr::new() generic

The previous signature for Instr::new() was too restrictive in that it
would create a new Instr from Op, but not from T: Into<Op>.

Fix that by introducing a generic version instead.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24998>

src/nouveau/compiler/nak_ir.rs

@@ -2395,9 +2395,9 @@ pub struct Instr {
 }
 
 impl Instr {
-    pub fn new(op: Op) -> Instr {
+    pub fn new(op: impl Into<Op>) -> Instr {
         Instr {
-            op: op,
+            op: op.into(),
             pred: Pred::None,
             pred_inv: false,
             deps: InstrDeps::new(),