[meta] Delegate finding the default value to OperandKind instead of its builder.

This applies both to the default_member value (which is now determined at
runtime, instead of pre-computed) and the rust_type value (which is
determined in the Operand's ctor, instead of the builder).

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

@@ -95,7 +95,7 @@ impl EncodingBuilder {
                         &inst.inst.format,
                         immediate_operand
                             .kind
-                            .default_member
+                            .default_member()
                             .expect("Immediates must always have a default member name set."),
                         immediate_value.to_string(),
                     );