Fall back to ugly debug

Merged: https://github.com/ruby/ruby/pull/13131

zjit/src/ir.rs

@@ -140,7 +140,7 @@ impl std::fmt::Display for Function {
                     Insn::IfFalse { val, target } => { write!(f, "IfFalse {val}, {target:?}")?; }
                     Insn::Return { val } => { write!(f, "Return {val}")?; }
                     Insn::Add { v0, v1 } => { write!(f, "Add {v0}, {v1}")?; }
-                    _ => { write!(f, "idk")?; }
+                    insn => { write!(f, "{insn:?}")?; }
                 }
                 writeln!(f, "");
             }