appendPlaceholder

fn appendPlaceholder(astgen: *AstGen) Allocator.Error!Zir.Inst.Index {
    const inst: Zir.Inst.Index = @fromBackingInt(@intCast(astgen.instructions.len));
    try astgen.instructions.append(astgen.gpa, .{
        .tag = .extended,
        .data = .{ .extended = .{
            .opcode = .value_placeholder,
            .small = undefined,
            .operand = undefined,
        } },
    });
    return inst;
}