[meta] Give a nicer error message when a legalization uses an incorrect number of arguments

cranelift/codegen/meta/src/cdsl/xform.rs

@@ -183,7 +183,12 @@ fn rewrite_expr(
     assert_eq!(
         apply_target.inst().operands_in.len(),
         dummy_args.len(),
-        "number of arguments in instruction is incorrect"
+        "number of arguments in instruction {} is incorrect\nexpected: {:?}",
+        apply_target.inst().name,
+        apply_target.inst().operands_in
+            .iter()
+            .map(|operand| format!("{}: {}", operand.name, operand.kind.name))
+            .collect::<Vec<_>>(),
     );
 
     let mut args = Vec::new();