unionInit

fn unionInit(
    gz: *GenZir,
    scope: *Scope,
    ri: ResultInfo,
    node: Ast.Node.Index,
    params: []const Ast.Node.Index,
) InnerError!Zir.Inst.Ref {
    const union_type = try typeExpr(gz, scope, params[0]);
    const field_name = try comptimeExpr(gz, scope, .{ .rl = .{ .coerced_ty = .slice_const_u8_type } }, params[1], .union_field_name);
    const field_type = try gz.addPlNode(.field_type_ref, node, Zir.Inst.FieldTypeRef{
        .container_type = union_type,
        .field_name = field_name,
    });
    const init = try reachableExpr(gz, scope, .{ .rl = .{ .ty = field_type } }, params[2], node);
    const result = try gz.addPlNode(.union_init, node, Zir.Inst.UnionInit{
        .union_type = union_type,
        .init = init,
        .field_name = field_name,
    });
    return rvalue(gz, ri, result, node);
}